Most aquarists have one or two plants that won't grow for them, so success isn't guaranteed, but in general, these are the easiest:
Java fern (regular, needle leaf, windelov, trident)
Cryptocoryne wendtii (Green, bronze, brown)
Floaters (duckweed, salvinia, frogbit, floating riccia)
Anubias (nana and petite would be best for a nano)
Pygmy chain sword (will need root tabs)
That should give you plenty to work with in a 1.5g. If it were my tank, I would replace the gravel with some cheap sand. Not only for the plants' health, but because it makes cleaning the tank much easier. Gravel has to be vacuumed, but detritus just sits on top of sand. You can siphon it out with some air line or even suck it up with a turkey baster - much easier than trying to maneuver a gravel vacuum around.
If you need scape ideas, check out this thread