Any of the common livebearers fit the bill, but IMO are too easy to be 'fun' to breed.
However examples would be: Guppys, Mollys, Platys
If it were me I'd want to try and breed Sparkling Gouramis in something that size.
Your main issues are:
Neons are more than likely going to eat any eggs that are laid.
The tank is very small, you need to be breeding very small fish and not many (which is a problem with livebearers)
If you have livebearers then you need to give the fry plenty of cover so they can hide once born.
If you have egg layers such as neons then you need somewhere else to house the eggs and/or fry otherwise the parents will eat them
If you have egg layers that play a parental role, then the tank will be 'ok' for awhile.
And in all instances if you want to keep a decent % of fry alive then you'll need a grow out tank...