I just put a few feeder comets in mine to start with, but Japanese trapdoor snails are a good choice to keep the algae down.
For more long term residence white cloud minnows is a good choice, like guppies, but no need for a heater.
Make sure to get some cover too, I am thinking I will get a bunch of duckweed to cover mine this year, in an attempt to thwart any raccoon or ? temptations.