We were there in March. Go to the "Old Lahaina Luau" (spelling probably incorrect). Although it is the only luau I've ever seen, my wife and I thought it was really great. We both agreed that it was the highlight of our trip.
Also, somewhere on the southwest side of the island my wife and I noticed a lot of cars pulled over on the side of the road at a beach, and a lot of local-looking people in the water snorkeling, swimming, etc. We went to Snorkel Bob's, got some gear, and had a great time snorkeling in a "local" spot. There are several places like this on the island, just look for a few cars. The locals know the best spots, so don't be shy.
Haleakela National Park is really cool, but you better be prepared for cold weather at the top. You gain a lot of elevation in the drive, and it gets cold and windy. If the weather is cloudy at all, skip this trip. You need a perfectly clear day, or the mountain will be entirely clouded over at the top any you won't be able to see 50 feet into the fog.
Drive the road to Hana. It will take most of a day, but the scenery is incredible. Stop often. The only drawback here is that the road is narrow and twisty, so if you get carsick easily you might want to think twice about this one. While you are on the north shore, you will be near the area where world-class wind surfers do their thing. Its fun to watch.