Amanda, I've visited two terrific nurseries in Mesa, both are A&P Nursery, just different locations. I found them to be very helpful and honest; they'll tell you if a plant is not right for the season or location. That said, here are my recommendations: Sunflowers, sunflowers, sunflowers!! They range from short and small to tall with mammoth heads. Best of all, they need nothing but water once they sprout. Other summer color could include lisianthus (very pretty lavender flower), Vinca, portulaca, gallardia, daisies, verbena. Boo's suggestion of purslane* is a good one, but I'd nix lantana; no matter how carefree a plant it is, sooner or later it will need some pruning and you can get a nasty allergic reaction to the sap.
* Edible purslane is wonderful in addition to the ornamental variety. Very, very easy plant to care for. Just needs water. ;o)
Good luck.