If you're into something more unusual you could have dwarf puffers in there, but they are not the easiest fish to keep so unless you have some experience or are willing to do a lot of learning fast, they might not be the best.
You could have one male betta, or five or six females, in that tank.
Pygmy corydoras are always an option, they are very cute and spend more time in the middle levels of a tank than their larger counterparts.
Otocinclus catfish are also suitable.
You could have some of the dwarf rainbows, such as Iriatherina werneri (usually known as the filament rainbowfish, threadfin rainbowfish but has so many names we usually use the scientific name) or some of the blue eyes (Pseudomugil species).
Endler's livebearer males are extremely colourful, you could have up to ten or twelve in that tank with good filtration.
Paradise fish - there are success stories of people keeping multiple males or males with females but personally I wouldn't risk it. They are colourful though and they don't need heaters.
Smaller tetras are perfect - neons, glowlights, embers (ideal cause you can have a big school in there, they are tiny) or rummy noses might be okay.
Galaxy rasbora or celestial pearl danio - two names for the same species, a delightful and colourful little fish.
White cloud mountain minnows are spot on, pretty and active fish.
Dwarf platies - make sure you get the dwarf variety as a standard platy may reach three or even four inches and would not be comfortable in a small tank.
Shrimp - Some are very pretty. Can also be added in a small group along with some fish to clean the tank.
African dwarf frogs - make sure they are DWARF frogs and not CLAWED frogs which get very large and eat fish.
Harlequin rasboras - the tank is a bit on the small side for them but not too small.
If you are willing to go slightly brackish (add a small amount of salt to the water) you could keep bumblebee gobies or some of the other small goby species, and still be able to keep guppies or blue eyes with them.