The question isn't so much what there is to do in Huatulco but what isn't there to do. There isn't another destination in Mexico that can offer such a variety of pursuits in one place, and there are very few places in the world like it. Incredible sportfishing, scuba diving, sea kayaking, whitewater kayaking and rafting, surfing, rock climbing, mountain biking, canyoneering, ziplining, golfing... it's exhausting just thinking about it all. Click on any of the image links to the left of the descriptions for more detailed information on each sport.
Scuba Diving
With the largest coral reef system in Pacific Mexico quickly dropping off into the deep blue, scuba diving offers both the opportunity to see tropical reef and deep sea pelagics on the same dive. Huatulco Dive Center is one of the best outfits in town for scuba diving, PADI certified and a great staff. They're located at Marina Chahue and run a nice new dive panga called Mako out of there. Caution about diving in winter - seasonal upwellings make for cold water and bad visibility a lot of the time due to nutrient blooms. While the swells are usually the calmest in winter, I've come to find that the best visibility is surprisingly during the times of year when there is much more swell, between April-November.
There are a handful of $500-$750 a day charter boats operating out of Huatulco, but like Cabo San Lucas, the bulk of the charter fishing is on pangas. Most of the sportfishing pangas come equipped with heavier reels for tuna and sailfish and the like, and a decent selection of large artificial lures. What they don't tend to have is any light tackle equipment, GPS, or depth finders. I notched up Huatulco's first IGFA world record fish on my boat, a 32 lb Gulf Coney grouper in 2012, so there are plenty of possibilities beyond the traditional surface trolling that all the local boats do.
The Rio Copalita, which forms the western border of Huatulco, is the largest river in the region and has a wide variety of whitewater levels to suit your taste for excitement. Check the Rivers page for detailed information about the various sections of the Copalita. For novice whitewater kayakers, the four mile highway-to-ocean section of the Copalita is a great learning area, with some fun little class I-II rapids and a lot of bird life. When the water levels are up, the Rio Zimatan can be kayaked, though much of it is expert class.
Hiking possibilities are amazing in the Huatulco region, yet largely undeveloped. Since the tourism market has been catering to less athletic and less adventurous clientele, activities that don't involve herding clients around like cattle have been overlooked. Semi-secret trails to empty beaches, gorgeous river hikes, and other trails throughout the mountains await anyone looking to get off the beaten path. In particular, Pluma Hidalgo has a beautiful easily accessible mountain ridge hike overlooking the town.