Just in Maui the beginning of June and I recommend much of what everyone else has posted with the addition of the Warren and Annabelle show in Lahaina. It is SO worth it. We read about it in the Maui Revealed book and posts here on Fodors and it was well worth the money. We typically like more of the outdoor activities, so this was a different choice for us.
The lounge show at the beginning is a little hokey (sorry Annabelle), but enyoyable and the food was very good. Warren, a magician, is simply fantastic and unbelieveable, and an extremely good comic besides. We never thought we would do a show in Hawaii other than an luau, but I would do this again in a heartbeat.
I believe you have to be over 21 (liquor served) to attend. A variety of ages were there and he really seemed to pick on the honeymooners. There are two shows a night, 5:30 and 8:30 and it only seats 75, so you need to make reservations early.
Please take this advice if you can swing it. You will not be disappointed.

1.Road to Hana trip(We liked Valley Isle tours)
2. Snorkel at Black Rock on Kaanapali Beach & Honalua Bay(like being in a fishbowl).
3.Watch the beautiful sunsets at the Hula Grill at Whalers's Village, with live music, enjoying delicious food & mai tais.
4. Take a drive on the north on the west coast of Maui to view the sites.But turn around when the road is not a state road anymore! It becomes very narrow, no guard rails & high cliffs. Not good for 2 way traffic!
5. Visit Iao Needle.
6. Visit Paia' & eat at Mamas Fish House.(The memory will live with you forever!)
7. Drive to Haleakala. Awesome
Enjoy!

Our favorite was biking down the volcano! We loved it!! We enjoyed Warren and Annabelle's also. Lahaina is a lot of fun to walk around and people watch. Maui is just great! You'll enjoy it no matter what you do!