Eleonora,
How about a small shoal of cardinal tetras. Or corys as big_fresh suggested. Or guppies (colourful and you could even breed them). As the tank is tall in relation to the width you will want to try and get a mix between top/middle/bottom dwellers. Corys will be mostly on the bottom, cardinals will be bottom/middle, guppies woudl be top/middle, etc. Have a look through some of the tanks in the Members Aquarium Pictures section and see if you see anything you like. Then you can have a look and see if thoise fish would suit a small tank.
I have two small tanks either side of my desk at work - one freshwater and one saltwater nano-reef. Some people in the office are fascinated, others are 'so what' and some don't even notice them (I even had to ask one guy not to lean on the over tank light fitting....).
Cheers, Eddie