Welcome, Suzie. Please let us know whereabouts you are - it's not really possible to recommend plants unless we know something about your climate. Your zone only will tell us how cold it gets in your average winter. We need to know how hot your sun is, and humidity, altitude etc. also have a bearing on what to grow.
A birdbath might make a nice planter, depending upon how rainy your location is. Unless your birdbath leaks, there will be a drainage problem for something like succulents, so if you live somewhere rainy, best to plant something that doesn't mind getting its feet wet. How deep is this birdbath? Plants might float out or rot if there was a long run of wet weather.
How about posting a picture of it, and also its surrounding area - are there sprinklers near it that will water the plants?
suzieroosie
