There are around 19 recognized dive sites around Bunaken Island, and close to 30 sites in all within the national marine park encompassing all five islands. Most dive resorts on Bunaken seem to do most of their dives around the island, which keeps the "commute time" to 20 minutes or so. However, it all depends on the weather and what is heard through the grapevine about what's been spotted around the park.
The diving around Bunaken itself is most wall dives. A wide sandy shelf surrounds the island, which transitions to a coral reef as it gets deeper, but then there's a sharp drop-off forming a sheer wall covered with coral, sponges, and a lot of critters. The "big" draws here are the sharks and turtles (the biggest I've seen) as well as the large schools of fish.
The wall extends down to 40 meters or more, making Bunaken a good place to learn Tec diving, if you're interested in that kind of thing. Currents can be swift and changeable around Bunaken, but the dive guides will generally keep you going with the flow and just drift along.