That is about 18.5G for the ones who use Gallons as a point of reference.
Take a look at Endlers if you are considering Guppies.
Angelfish are pretty but would prefer a larger tank, and multiples can cause HUGE, I mean huge problems. Not recommended.
Dwarf Cories - Pygmy and Hastatus are nice, get 6-12 of them for a fun bunch of activity.
Ember Tetra, Chilis aka Boraras Brigittae, one of my new favorites are Neon Green Rasbora aka Microdevario Kubutai
These are mostly smaller fish as you can have a nice school size but since your tank isn't really large, you would be limited with many commonly available fish for stocking the tank.
Dwarf Gourami / Flame Gourami (only 1 fish of these)
After you start seeing actual types of fish you like, check tank water temp and PH parameter requirements and then whether they will get along with each other and how many of each to get if they need friends.
It is best to pick the fish you like best and build your tank around that.