Rent yourself a kayak and some snorkel gear and spend 1/2 day at Kealakekua State Park, a marine park that is actually the bay.
It's about 1 1/2 miles across the bay to Captain Cook Monument and you'll find some of the best snorkeling in the state their.
While paddling across the bay, be sure to look for the spinner dolphins.
The dolphins were all around us the day we went out. They're fun to watch but hard to get a good picture of (at least if you're using a disposable waterproof camera).