If you use Zinnias, use the Profusion types. They are less "single flower
-ish" for a lack of a better way to phrase it and make more of a mound which would probably cascade, at least a little. I don't think Thumberlina would give you the effect you're looking for unless you use just one in the middle or something.
Those Wave-style (including Tidal Wave) Petunias are excellent, especially if you can bring yourself to pinch them every so often. They do well when they don't get watered daily which I think is a plus.
Look for Bidens or dahlberg daisy or quick, get some signet marigolds and start them from seed
-- they come in orange, yellow or burnt-red and will cascade over the edges. They do not look like, nor smell like regular marogolds....the foliage it feathery and the habit is lax. They look good peeking out between somehting else if yellow-orange-red fits your color scheme.
I have some other weirder stuff I'm trying for the 1st time this year, but I honestly can't recommend anything yet: Cuphea (2 kinds) and a bachelor buttons repens which will peter out in the hot weather, but should make an attractive pot, if not hanging basket.
I have also heard but not tried some of the vines.
I think Njoynit said Nasturtiums above, and they are always wonderful, especially in a pot where you can control the amout of fertilizer (none) they get. They get really big, but there is a miniature one you can buy seeds
of called Princess of India, I think it is a red like Empress of India, just smaller, and also one that is really easy to find of mixed colors called Jewel Mix.
I don'd have places to hang a lot of baskets and stuff, but I do have space for pots and containers -- both big and small, so I have a lot of things planned for those pots. Right now I have about 50 jugs of flowers
that have germinated, but they only have seeds
leaves, so I am a ways away!