Box turtles and diamondback terrapins have great temperaments, even wild-caught specimens. We found one once that was so docile, we were convinced it was someone's pet. You have to be careful with terrapins, though. They're highly endangered here in NJ, so make sure you're not buying WC.

CB painted turtles and red-eared sliders can be pretty docile, but they're kinda hit-or-miss. The Eastern painted girls at my school are great, but I had an Eastern painted male that was pretty nasty.

With most chelonians, though, you'll probably have to upgrade to a bigger size at some point.