My first thought would be some geraniums, check out your garden centers there will be alot in bloom by then, I was just in our local Adam's the other day and they had alot of stuff in bloom already. I was good I only bought pansies. Good luck

Maybe add in some dwarf marigolds to add contrast with the geraniums. Petunias in hanging baskets, window boxes or even in the ground would add a punch of color too. I am probably not much help because nearly every thing I grow is a nectar plant for hummingbirds.

Thanks so much- I'm heading to the store today since it's so nice out! Hooray!