you have a few options...
you can pick 1 off this list:
school of 6 ember tetras
school of 6 Rasbora merah, nana, or brigittae
school of 5 pygmy chain loach*
a single male Dwarf Gourami
or Honey Gourami
a single male betta
trio (2 females 1 male) sparkling Dwarf Gourami
trio (2 females 1 male) Badis badis
, or Dario dario
2-3 male guppies or endlers
Except for the gourami and betta, you can add a handful (3-6) of shrimp (red cherry or ghost are good) and 1-2 nerite snails...
* are somewhat sensitive species... Stay on top of water quality.
A small tank can turn out really well... I'd definately encourage you to go planted though...These are some easy plants of suitable size-
cryptocyrne lucens, parva, petchii
narrow-leaf Java Fern
small marimo balls
You could leave it open top and add a Echinorodus 'Kliener Prinz' or small Aponogeton crispus
in the back as a centerpiece..
Dry-start method could make the tank much easier to set up planted... It's essentially where you add the substrate (with a tiny amount of soil mixed in), add rockwork and wood, add the mosses and some select plants (like hairgrass, glosso, or dwarf baby tears), and keep covered with plastic wrap. The plants grow faster in high-humidity than they do submerged, and you can get a really nice tank with 'difficult' plants the easy way. Mist with weak fertiliser mixture every day or every other day.. it only takes 2 weeks or so, but you can let it go as long as you like that way.