I have one nano with a dwarf puffer (needs to be the only fish in the tank) and another with shrimp and snails. Those are both fun choices.
I have anubias petite nana in both. My 5 gallon has java fern, riccia and a crypt and my 2.5 gallon has crypts, some small-leafed stem plants, dwarf sag, and microsword.
Those are just a few things that have worked for me -- I'm sure you'll find your own style that works for you.