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I am in the same situation as you. I like to plant petunias in some of my planters. They always seem to do well. They're easy to grow, and I think they have such a nice scent. especially on warm summer evenings.
I have also grown geraniums in planters.
I like to choose plants that are relatively easy-care, just because it makes things simpler for me. If you buy seeds, be sure to check the back of the packet to see how tall the plant will get. IMO, any flower that gets more than 12 inches tall will look a bit ungainly and may tend to flop over. (The exception would be if they were planted in large planters. I have some of those, as well, but in this post I'm speaking only of the oblong-style planters.
Two combination I particularly enjoy are white alyssum interspersed with yellow marigolds, and dark blue petunias with yellow marigolds. (I'm not fond of the color orange, but that's just my personal preference.