For 2.5 gallons, a betta is all you can have. Anything else will be overstocked and too cramped.
I disagree, many fish under 4cm would be ok. Dwarf pensil fish or other small characins would be ok, as would small livebearers.
However, none of these are what I would have, I prefer having a few different types of small snail amd very heavily planted. I have a 3gal with a tiny heater and filter, sand, a light and thats it for tech, but a bet my plants are more healthy and vibrant than in a high tech. Have a look on ebay or snailshop.co.uk for some small suitable snails.