Maybe New Guinea impatiens? Without seeing the bed, it's really hard to guess. Of course, anything you put in there will need lots of water this time of year!

Zinnias. From seed or pots if you can find them. Cheap, fast, colorful.

It's difficult to grow things under maples because their roots are near the surface and they steal all the water. August being so hot and dry to boot, I'd get some really big pots, then keep them well-watered. You can grow just about anything your heart desires if it's in a pot!

Thanks everyone - couldn't find zinnias so went with some marigolds and impatiens - must have worked b/c in one day we had 2 women who want the apartment (painting, cleaning, new stove and countertops didn't hurt either). Regina, we have some pots on the porch which we've got flowers in but I bought some alleged "soil amendment" mulch to put around the flowers in the beds and they seem to be doing all right. Hopefully our neighbor will remember to water them while we're gone! Thanks again,
Karen