IMO, I wouldn't drive it twice. Just find a beach to camp on or hit up a bed n breakfast in Hana. Its a LONG drive on a SHORT vacation on this island. Charles Lindberg (sp?) is buried out there, plenty of hiking trails too. I would definately stop at the 7 sacred pools, they are great for swimming if the water isn't running high. There are some really cool black sand beaches to see, we usually stop on one for a picnic lunch. If you like bridges there are 50+ single lane stone bridges to cross, and during the rainy season most have water running under them. If you want to see some plants and animals there is a cool spot to stop and check out. I can't remember the name, but its 10 per person I believe, and you can't miss it. There is also a view of the rock featured in jurassic park when they are flying towards the island in the helo (not very impressive IMO-just a rock in the ocean).
If you got up Haleakala to the summit you'll see our toys - all the telescopes and whatnot I get to play with from time to time.
As far as the rest of the island, the only things I would say you need to do is snorkel and hit up a luau. They have snorkel trips out to molokini, but honestly you can get into some awesome snorkeling from shore if you don't want to spend the money. Eat at Pizza madness in Kihei and cheeseburgers island style at the shops in Wailea. The food in Lahaina didn't impress me at any of the places I've eaten, but thats my opinion. If its raining and you want sunshine, come down to Kihei since it rains about five times a year on this side.
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