Kite boarding off the west coast, down around the Atlantis dive site I believe. Looks fun (if you're fit and athletic, I imagine).
Wind Surfing off the east coast, in Lac Bay, an area call Jibe City, IIRC.
Drive through the Donkey Sanctuary.
Take the long way tour through Washington-Slagbaai Park up north - hilly to mountainous. Take the receipt for your marine park tag; when I went, it saved the $10/person entry fee. There's a little museum at the entrance. Get out at Playa Chikitu and watch ocean waves pound the east coast. A fairly scenic area for outdoor wide-angle natural photography, if you're creative. You'll see some interesting rock formations.
Follow the coastal road along the west, south & up the east coast. Drive across and around the island a bit; hilly up north, very flat down south, arid, scrubby, and up north huge numbers of thorns.
If you're into bird watching, there are some to see.
Shop around the capital; little jewelry store called Jewel of Bonaire is a personal favorite, such as for sea turtle pendants with a beautiful opal, and while they don't give them away, a couple of nice ones were outrageously expensive, either. They've got pricy stuff if you want it, but you can get a nice memento without getting ridiculous.
Some people hike or bicycle; be mindful that while arid, it's hot and very sunny. Speaking of which, I haven't done this, but I recall a parrot rescue related to the yellow-shouldered Amazon parrot native to Bonaire, and a search turned this up -
EchoBonaire.org, and looks like they do tours. Any they've got volunteer opportunities, if that interests you.
Another approach to take is go to a cruise ship website and look at their excursions section for Bonaire and see what they offer. Or Trip Advisor's 'Things to Do' for Bonaire. That'll show you some options.
Richard.