While staying in Ubud, I hired a car and driver, and spent some time traveling around Bali, all for $35 per day including gas. Split with my two Swiss friends, that’s about $12 per day to be driven around to wherever you want to go, much cheaper than taking a pre-arranged bus tour.
Our driver was great fun too – his full-time job was actually managing an IT / electronics shop, but his brother (who managed our homestay in Ubud) roped him into coming to give us a tour and traveling around Bali. Surprisingly, it was the first time he had toured Bali, so he had a whole ton of fun.