On Maui, you could do the downhill mountain biking on Haleakala.

Snorkeling at Molokini off Maui.

My friends and their kids took the Trilogy boat trip to Lanai and loved it.

http://www.sailtrilogy.com/

On Maui we have done a kayak trip, it was a half day, but was a lot of fun, we had a group of 6 with a couple of teenagers in the group. They provided snorkling equipment and had some nice time in the water as well as on the kayak.

We have done this two times, once near Wailea and the other on the west end. Both were really nice trips and we stayed at the beach afterwards and enjoyed the beach.

I don't remember the outfitters, and we used the same one both times.

We also did a parasailing afternoon and although this only takes a couple of hours, was really fun and a nice boat ride when we weren't up in the air.

I also think the Haleakela bike trip would be good for this age group. They will have to get up early, but it is a good ride. My sister did it with the same group of teens we had with us.

On Oahu, a catamaran ride from the Outrigger Waikiki on the Beach catamaran would be fun.

We're leaving for Oahu and Maui tomorrow with our 2 teens, 19 and 15. This trip we are planning on a lot of beach and pool time, and just relaxing, with a couple of scuba dive days thrown in. The kids are excited that (for once) "vacation planner Mom" doesn't have a daily itinerary of activities!

Have fun.

Carole

On Oahu: Don't miss the Arizona Memorial and Missouri, snorkeling at Hanauma Bay, the walk up Diamondhead, and the drive around the island to Sunset Beach for sunrise, stopping at Lanikai and some of the other beaches along the way.
On Maui, there's good snorkeling on the less-crowded side of black rock at Kaanapali Beach.

I'll second Trilogy to Lanai for the day.

Sunrise at Haleakala/bike down.

Snorkel with the manta rays.

Golf, horseback riding, "leave me alone - I just want to sit on the beach by myself".