My daughters aged 14 & 16 are coming with us to Hawaii; they're not exactly couch potatos, but do take a lot of convincing that something else can beat TV. Any suggestions beyond the beach?
Leave them at home with the TV. Use the money you save by not paying for them to go on holiday to pay for a nanny to come and stay with them from dinner time until the morning when they go to school.

As soon as they are there in Hawai'i , suddenly the TV will look very dull indeed.
Now, it does depend on which island you will be on. There are so many cultural events, dance lessons, hiking opportunities, farm exploration and such. But it does vary on which Island you will be going to.

head over to the North Shore, easy to get to, to see the famous surf beaches and surfers and places they filmed that surf movie with the, umm, surfers! Blue something, it was a pretty cool flick for kids, maybe hire it before you leave home
Take them around to some caves where maybe theyll find a strange polynesian idol that bestows them with a curse they have to get rid of by casting the idol into a fountain of lava somewhere on the Big Island. The curse may involve a hairy tarantua and getting molested by Don Ho.

Om, do like my parents did, no tv during the middle of the day on weekends. That forced us to do other things... outside. This way they will already be trained to not watch tv in the first place. Besides, why give them a choice. Tell them what they are going to be doing for fun, and then make them do it.

Blue Crush #3.
If you are going to Hawaii why are you looking for suggestions "beyond the beach" for teens. Days and days of entertainment right there. Surfing tandem on 10ft boards at Waikiki and trying out the waves on the north shore if it is not too big, snorkelling, wandering the north shore looking for turtles (we also came across a sunbaking seal), sunbaking and checking out the boys/girls on the beach, sailing on a cat offshore........
You could also buy a ukelele (cheap) and get them a few lessons to impress their friends when they get home, ditto with hula lessons, shopping, playing surfing Monopoly (you can get it from the Billabong shop in Waikiki).......