travel : sightseeing general
There are a ton of things to do in Hawaii. Here are some suggestions to help give you ideas, this is by no means an exhaustive list. If you would like to bring a guidebook, I suggest "Oahu Revealed". I used the Maui Revealed book and thought it was great. It's like a local friend wrote you a guide book with "locals only" knowledge.

Waikiki and Honolulu are very urban, so you may get lost. If you have a portable nav system, I would bring it, just please don't leave it in your car and ask for it to be stolen. If you ask for directions, please note that most people only know street or highway names, not the route number. So if you ask them how to get to Route 61, they won't know what the heck that is, but they will know what and where Pali Highway is. The only exceptions are H1, H2 and H3 freeways, they don't have any other names. (Yes, yes, I know... How can it be an interstate freeway when it doesn't go between states and actually only is in 1 county.) :) One other thing, people sometimes give directions based on landmarks, not North-South-East-West. If they do, the 4 main directions are: Mauka (mountain), Makai (ocean), Ewa (towards Ewa, usually West), and Koko Head (towards Koko Head, usually East).

Several people have asked about the neighbor islands. In general, they are less urban and populated than Oahu, and since this is what I think most people are looking for when they imagine Hawaii, I recommend going to a neighbor island if it's feasible for you. I think Oahu has a lot to offer but sometimes it can be harder to find the secluded beaches or undeveloped hiking trails that are easier to find on the neighbor islands. Maui is very popular and has a lot of great beaches that are easy to find and the extinct volcano Haleakala. It can also get windy sometimes but lots of people love vacationing here. The Big Island (Island of Hawaii) has 2 sides: Kona which is dry with a lot of black lava rock and site of the Ironman, and the wet and more suburban Hilo. The Big Island also has the only active volcano in Hawaii (Kilauea in the Volcano National Park) and several observatories. If you're lucky, the lava will be flowing in an accessible spot in the park, and you can hike out (day or night) to see the lava flow into the ocean. Kauai is very lush with a lot of rain forests and valleys and probably the least populated/developed of the 3 most popular neighbor islands. Hiking and kayaking here are popular activies.

Have fun, and where ever you go, don't forget to bring water, sunscreen and your camera.

Beaches
General
There are a ton of beaches on Hawaii and hopefully you can find one that fits your tastes. Before going, I suggest bringing something cool to drink and bento boxes or plate lunches if you plan to stay there during lunch. And of course your sunscreen. Even if you think you don't need it, please apply some, I've seen countless lobster-red tourists. Please don't leave anything valuable in your car. Rental cars are very distinct and often get broken into when parked in beach parking lots. Leave valuables in the hotel safe or at home. Bags and purses are generally safe on your towel, so you can keep your car key, hotel key and camera hidden away in there. Also, please don't underestimate the power of the ocean. If there are warnings posted about high surf or currents, don't take it lightly. Finally, beware the Portuguese man-of-war (stinging jellyfish). If they are around, usually there are signs posted, but sometimes a stray one can float around. Just don't touch the trailing tentacles of any jellyfish, even if they are dead on the beach you can still get stung.

Ok, so now that I've gotten all the warnings out of the way, let's get to the good stuff. Below are some beach suggestions and descriptions. You can also see the favorite beaches of Hawaiian Airlines employees. Their site has broken it down into "Family Fun", "Active Adventure" and "Serene Seclusion". Please note that some of the beaches are on neighbor islands.

Waikiki Beach
It needs no introduction. It can get crowded, but the sand is nice (and cleaned every night by machines) and the waves are small, so it's a great place for small toddlers. There are also a lot of surf stands that offer lessons. It's a good place to go for lessons since the waves are consistent but small. Plus, there are a lot of surf rental/lessons shacks.

Waimanalo Beach
Located on the South-Eastern shore, it's south of Kailua but north of Makapu'u. It's pretty popular with families, both locals and tourists. The sand is nice and fine here and the beach is pretty wide. The waves are pretty calm here and the beach slopes gently down (versus dramatic dropoffs at some beaches). There are also picnic benches and campgrounds (permit required). Some scenes from "Lost" remind me of Waimanalo, the ironwood trees (kind of like pine tree) are distinct to this part of the island.

Lanikai Beach
Located in Kaneohe, it's hard to find in a residential neighborhood, but worth the trek. You need to park in a residential neighborhood and find the public footpath to the beach. It is smaller but has nice views of 2 little islands and the sand is nice and the water is usually calm. It's also usually not as crowded since it can be harder to find. But there also isn't any water, public toilets or lifeguards.
Click here for pictures and directions to Lanikai and nearby Kailua Beach.

Hanauma Bay
An underwater preserve, it's a popular place to snorkel. The fish are very tame and will swarm you hoping for food. It encloses a huge corral reef so the snorkeling is great. It can get crowded and the parking lot fills up early. I suggest going early. Also, unlike most beaches, there is a parking fee and a separate entrance fee (per adult, kids under 13 free) and they are closed 1 day of the week (currently Tuesday). Snorkel gear is available for rent at the beach but is slightly more expensive.

Waimea Bay
Located on the North Shore, it's pretty famous. In the winter, waves can get huge and they hold world-class big-wave surf competitions here. But in the summer, it's often very flat. The water is very clear here and there's a nice lighthouse and some rock outcroppings where kids often jump off. If you go, I suggest making a driving tour of the North Shore and hit other North Shore sights.

Sights and Places

Diamond Head Diamond Head is an extinct volcano, but has a great hike that leads to some of the best views of the island. From the peak, you can see Honolulu, Waikiki, Kahala and most of "downtown" Honolulu. On clear days you can even see Molokai (neighbor island). Most people will be able to do the hike (paved and packed dirt trail with hand rails on most of it). There is a tunnel section, several long sections of stairs and 1 big 3-4 foot step/climb at the end, but entirely doable for most. There is no water, so bring your own, most of the trail is exposed to the sun and it will be HOT, so go early if possible. The hike can take about 30-45 minutes one-way. Bathrooms are at the start of the trail, nothing along the way or at the top. There is a small parking fee, but it's well worth the view. Don't forget your camera!

USS Arizona Memorial, Pearl Harbor
Still a working base, you can visit the sunken USS Arizona and the memorial that floats above it. Before the short boat ride to the memorial, you watch a short film talking about the attack on Pearl Harbor. Everything is run by the US Parks Service and while tickets are free, donations are appreciated. Tickets are on a first come, first serve basis. You show up and get tickets to the next available tour. There is a bookshop and some displays to see if you end up having to wait.

Tantalus
Name of a winding street in a residential section of Oahu that has nice views of Manoa and Honolulu. There are several unofficial lookout points along the drive, just look for wide turnouts where the trees are cut back to allow for a view. At night, it's also something of a "Lover's Lane".

Pali Lookout
Dramatic cliff overlooking Kaneohe where King Kamehameha's warriors threw the defeated Oahu warriors off the cliff, and finally united all the Hawaiian islands.

Iolani Palace
Palace of the Hawaiian Monarchy. You can take guided tours to see the inside and learn about Hawaiian history. The interior is sparsely furnished and the rooms are pretty modest, so if you're waffling about the price, don't go. Sometimes the Royal Hawaiian Band will play in the bandstand on the Palace grounds and you'll see a lot of office workers eat their lunch and enjoy the free concerts. Even if you don't take a tour, it's a great place to wander around. The facade was used in "Hawaii 5-0" and I think it was the police headquarters. Nearby there are a bunch of other sights. Next door is the State Capitol. Across the street is a famous statue of King Kamehameha. One block away is one of the oldest churches in Hawaii, Kawaiahao Church, and some buildings showing what early missionary homes where like. If you want to learn more about Hawaiiana, check out Bishop Museum. They also have a cool observatorium.

Polynesian Culture Center
Located on the North Shore in Laie, it's a huge park that displays the cultures of several Polynesian islands, including Hawaii. There are shows by entertainers in native costume, including hula, fire dancing, Tahitian dancing and other crafts. They also have a luau and are open at night. It's good for families as there are also a lot of instructive activities for the kids. I haven't gone since I was a kid and we went on a school field trip, but I do remember there was a lot to see. It's pretty far from Waikiki and downtown Honolulu, so if you don't want to drive, they have coach bus service to and from Waikiki.