Some kind of floating plant, and possibly some kind of thick rooted plant. It's going to depend on what you can keep alive, really. Are you just looking for a simple plant, no added CO2 or extra light? Hornwort seems to be a popular choice amoung Livebearer keepers though. And it's pretty easy to keep alive.
Also, Guppies are Livebearers so there will be no eggs.