Intrepid offers really cool tours along this vein. I especially like the Fiji one. You spend 14 days on the island, volunteering to coach a children's sports team (I would do swimming), and then you spend your free time at the beach or pool.
Fiji is very appealing, but I fear this trip may be a little too unbalanced (i.e., too much volunteering and not enough relaxing). Also, I think we would be traveling with lots of eighteen and nineteen year-olds who are on their gap year between schooling. And there's dorm-style accommodations. Okay, never mind.
Another good source for do-good honeymoon inspiration: this post from Daily Candy (thanks, Jean!). Through Hands Up Holidays, we could go to the Yucatan to help the elderly, while also having time for snorkeling, kayaking, biking, etc.
But maybe the volunteer thing is too much during a honeymoon. Maybe we should look for some sort of classes instead? Jordan over at Oh Happy Day is going to Hawaii for a film photography class. Hmm...