Batur lake is a place where Bali converts to a Europe. The lake looks like a lake in the heart of Swiss Alps, you easily can find a view that no one will recognize that you in tropics.
First of all Batur lake is located in the north west corner of Bali, far from all tourists places, so be prepared to a long ride. Ticket to enter to this place costs only ~2,5$, it's pretty cheap for so beautiful views.
When you start go down to the lake on the right side of a road you'll see a small cafe. You definitely should visit it. From this cafe opens one of the most amazing views.
During go down you'll see a lot of clinker, unique sight. Last eruption was not so long ago in 1999.
We found a two good photo spots on the lake. First on boat station, the whole landscape is beautiful, but don't look on water near dock, it'so dirty :(
Second, good photo spot located near Hot Springs. We avoided going to pool with hot springs, it's very expensive for non-residents. To catch a good view you can go a bit right from hot springs, and see old abandoned house near a lake. The view from there is excellent.
Be prepared to a lot of beggars. It'll be really hard to leave Batur lake without trinket. You can ask beggar pretty anything, we use their as free or very cheap guide.
Be careful going back, turns are steep and visibility is poor.