Did almost the same as you but went to the Big Island instead of Kauai.
Pearl Harbor, obviously. Get there first thing in the morning or you'll sit half the day waiting for your turn. They only hand out a limited # of tickets per day. You renting a car on Oahu? We circled the island after Pearl Harbor. Pali highway > Kamehameha highway. Pali overlook had a great view. Byodo-In temple was interesting, but only stop if you are driving by, don't plan a trip just for that. Sunset beach (awesome plate lunch up there, can't remember the name of the place.) Try a plate lunch somewhere. Shave ice is good too. Dole plantation if you're passing by. There isn't much worth doing there, but I'd fly back to Hawaii just for the pineapple ice cream float I had.
Stay away from route 93 and the west side of the island, that's the local side. While I only had one bad experience with a punk teen local, everyone says they don't want you in their spots. You get a vibe if you pull up somewhere you're not wanted, don't ignore it. Lots of homeless in lesser used beach parks, stick to the big tourist beaches.
On Maui we spent a lot of time on the beach but had two sightseeing days. Haleakala sunrise was awesome (wife was cold). If you go, dress very warm. 40 degrees doesn't sound cold but after a week of high 80's it hits you like -40. Road to Hana was fun on the way down, sucked on the drive back. If I did it again I would take a guided tour. The driver doesn't get a lot of scenery as you must pay attention to the drive. Let the locals pass as soon as you have a pullover, remember they are just living their daily lives. Had a good dinner at Willie Nelson's bar in Paia after the drive.
You staying in west or south maui? Snorkeling black rock (Kaanapali beach) was fun, rented equipment and went there almost every morning. Lahaina to satisfy the wife's shopping. Iao valley was cool, especially after a heavy rain with waterfalls coming off the cliffs. Get some Maui onion rings, another thing I'd fly back just to eat.