Istanbul, we toured with the Mini 15/20 seater busses. You can see so much within a group since group entry fee is included to most attractions:
Athens is another city easiest to tour with the 2 day Hop on Hop off Buss. Our accommodation was close to the Acropolis. Best time to visit is the late afternoon as lines are shorter and the attraction isn't over populated.
Best way to tour Vietnam is with a small 15/20 mini bus: