I live in Indonesia, and I often go diving to Sulawesi.
Gangga island is a superb place, and your girlfriend will love it, for sure. Snorkelling is good, dives around here are great,lots of soft coral, and lots of rare things (frog fish, leaf fish, pigmee sea horses,...). Accomodation and food are first class.After one week diving in BUnaken from Murex, you might appreciate the change.
By the way, Manado is an uggly and dirty town.
On Siladen, prices will be more or less the same as in Gangga. But you will be in the middle of BUnaken Natural park...
If you want to stay onshore, try Kikisi Nature Resort (
www.kikisi.com) Nice bungalows, nice beach, 15' by boat from Bunaken.
Still onshore, Santika hotel is nice, and has an excellent dive center (Thalassa, 5* PADI center), but no beach.
The resort on Bangka is nice, but very remote. No shops, no restaurant, just the bungalow and the beach. It's a post card. Same dives as from Gangga island.
Finally, in Bali, be careful about who you dive with, and where you dive. Some spots are potentially dangerous, and some dive centers are even more...