The thing to do in Bunaken is dive, dive and dive. However, the steep walls and occasionally strong, rapidly changing currents mean that many sites cater more to the intermediate/advanced diver. There are beginner-friendly sites too and all dive shops can arrange introduction dives and Open Water Dive courses.
The North Sulawesi Watersports Association (http://www.divenorthsulaw...) offers oodles of detail on diving in the park.
All dive shops in the park are affiliated with resorts, so see Sleep below for listings.
Snorkelling is fantastic in front of many of the resorts around the island, with an incredible amount of marine life inhabiting the shallows. Remember not to snorkel without fins as the currents can sometimes be strong and change quickly even when they are not. Pick a reference point on the island and do not stray too far unless you are a very confident swimmer.
