Umm, besides diving you have a choice of kite boarding on the southern end of the island or windsurfing next to the nudist resort on the west side. I believe there are one or two all inclusive resorts on the island that may have a few activities, but for the most part, I think you're on your own to create fun.
Bonairetalk.com, like amazon mentioned, is probably your best place to visit in order to get more info on various activities. I've only been once and we stayed at den laman, a little 20-25 unit condo complex right on the water a little north of town.