Snorkel Bob's is your friend.
Go to the Old Lahaina Luau. Its a lot of fun.
Black Rock had some good snorkeling, but look around as you are driving around the island for groups of cars pulled off the side of the road. These are locals, and they know they best snorkeling spots. We stopped at one and saw all sorts of fish, many types of which we didn't see at Black Rock.
The road to Hana is fun if you like scenery and winding roads. We were there this past spring, and the waterfalls had a lot of volume at the time.
Mama's fish house is good, but as mentioned, very expensive.
While near Mama's, check out the nearby beaches for world class wind surfing.
Haleakala National Park is really nice, but make sure you check the weather first. Our trip to the top was not as good as it could have been because the cloud cover made sure you couldn't see more than 50 yards in any direction.
Try the breakfast buffet at the Maui Sheraton. You won't regret it.
For Honolulu, just go to Waikiki and walk around. You are sure to find something fun to do with all the restaurants, clubs, shopping, etc.
Pearl Harbor is neat if you like historical stuff.
You can hike into Diamond Head.
Go to the north shore of the island for Pipeline (which should be really big this time of year).