If you stay on Bunaken, you can ask your dive operator if they make any trips to Bangka. Because of the distance they may only go when they a minimum number of divers and charge a fuel surcharge. (as it is about an hour compared to the local dive sites which are just minutes)
Take long safety stops in Bunaken, as often you will see big fish near the surface, Napoleon wrasse can often be spotted from the boat. I have seen dolphins while underwater (with scuba) but that is very rare. Small groups of Tuna (just three or four) and big bump head parrot fish. Big turtles. Occasional sharks (usually down deep) and eagle rays.
A few years ago a baby Whale Shark (about a month old I was told) was seen off Bunaken. Very rare, But you never know.
From the shore you may see Dugongs (relative of the Manatee). While diving off Bunaken the guide from another group told our guide he had just seen a dugong underwater, but we did not see it.
There are several low cost options on Bunaken. Be sure to ask all dive operators what their polices are in terms of dive length (they use Aluminum 80s with yoke valves) and diver to guide ratio.
Two Fish Divers (
Dive Manado/Bunaken and Dive Lembeh with Two Fish Divers Bunaken) has several budget rooms (some with attached bathrooms and some without) in addition to stand alone cottages on Bunaken (they have a smaller operation on Lembeh) .
Some of the smaller operations that have several small boats may allow you to have you own boat and make
your own schedule if you are in a small group (e.g. 6 or 8)
Do as many night dives as you can in Lembeh, its great fun.
Bring a small light for daytime use, as you will want to look into the crevices caves and under ledges on Bunaken's walls.
At night,
do NOT walk barefoot. Bunaken is known to have
millipedes and the bite is very painful.