I have purchased two cases for my G12.
The larger one is a Lowepro product, model Rezo 110 AW. It is able to hold the G12 (with its lens adapter and attached UV filter fitted onto the adapter ring), the attached wrist strap, the Speedlite 270EX with an attached diffuser), a Nikon lens cleaning pen, and the Canon remote switch. I am sure that a charger and spare battery will fit into the case. There are small compartments into which I place notes and instructions that i printed for myself.
The wrist strap is Economy Belt ProStrap from ProStrap. I ordered it from CCNow.com I like it; the Canon wrist strap does not fit into the slot on the camera -- I had to return it.
The smaller carryinbg case is leather, and just holds the camera. I bought it from AgreedStore
It is very nice, and I am pleased with it.
Hope this information is helpful.