Chinese Elm or Tallow. Maybe a Chinese Pistachio (hmmm, not much of a theme there).
I have a japanese maple in a pot and its truly stunning, but a bit temperamental - that's probably more the Perth climate though. I would also never consider putting it in the ground but again, more our climate than the tree perhaps.
Claret ash
Jacaranda would be a great one too.

My grandparents used to have an enormous almond tree in their backyard that was great to sit under (or climb!) and even better were the almonds we got every year.

I agree with ausgirl that anything you want as a shade tree is probably going to be too big to have in a pot for any length of time at all. One of the secrets to beautiful big shade trees is getting them in the ground and establishing a good solid root system as early as possible.