I loved panseys and snapdragons as a kid (still do really, but then I am a big kid!) My snapdragon (can't spell antirinum) have self seeded all over the place and are up already, so that can't be hard to grow!
Or maybe mini sunflowers? Tall ones would obviously be too big for pots, but little ones, I'd have thought, would be fine.
Or something edible - some herbs. Basil, parsley? Strawberries? Although they probably wouldn't fruit until next year. lettuce? radishes?
The more I think about it, the more there seems to be!