There are small, adjustable heaters, that will fit a 2.5 gallon aquarium, but will look very out of place due to their large size. I would recommend you get a pre-set heater (usually pre-set to 25.5 C or so).
As for fish, I would say maybe a single Betta, or perhaps a small school of Microrasbora.
all fish will jump out of the tank if it has no top(bettas and schooling fish especially), and why clog the tank with a heater if you can place one of those terrarium heaters under the tank and conceal it well. set it to the right temp and youll have a nice setup thats free of that annoying heater look.