Well, what kind of fish do you like? And what kinds of plants do you want? There are a lot of possibilities!
I have a few small (10-20g) planted tanks that go something like this:
-dwarf pufferfish: 3 puffers, lots of plants and rockwork, and plent of snails for the to eat all day long.
-tetras & corys: small schools of candinal and rummynise tetras and a 7-piece school of panda corys. Lots of plants and some cherry shrimp
-rasboras & corys: a school of scissortail rasboras, a school of pygmy corys, lots of plants, and a couple biiig snails.
You can go a lot with a 15g tank.