I used to dive with Minahasa Divers when i was there few years back. Used to stay in Tasik Ria but i think they operate from Hotel Sedona now.
You can visit their website for more information:http://www.minahasadivers.com/
I think you should definately make a trip to Lembeh if you have the time.
Tried night/ evening diving theree as well. Can't really remember much but think it was good. YOu should do at last 1 night dive since ur there.
There's not much activities there except eat and massage but im not sure about now coz i was there abt 4 to 5 years back.