Don't underestimate bettas. Some are absolutely gorgeous and make a very fine specimen for a little tank.
As far as "other" fish are concerned, if you have some driftwood available, I'd consider 1-2 oto cats. I've had 3 in a 30cm cube for over a year without any issues whatsoever. Well....like a week ago I was cleaning the glass and bulldozed one off with my algae magnet. He did fine for about a week but just yesterday I found him in the intake strainer of my aquaclear 20
But that wasn't the tanks fault, all me not paying attention. I would however advise to not put these guys in a fresh new tank. Give it a month or so for the cycle to complete and biofilm to be readily available for them. These guys do have a tendency to be somewhat fragile though.
Just keep on top of your water changes (nutrient export/replenishment) and you should be fine with 1-4 nano fish (tetras, guppies, oto's, bettas [just one] etc.)
Some shrimp with a couple oto's would look nice.
Also, with wood in the tank, I've never directly fed my oto's anything ever. They survive solely on micro algae/biofilm the tank produces and the wood. Going on about 18 months now.
If you were doing saltwater I'd tell you to get a juvinile occelaris clown and upgrade to a 10g. You can do a lot with 2.5g of freshwater if your husbandry (how well you take care of your tank) is frequent and productive (actually maintaining your tank). Just limit your fish load to species that don't exceed 2" (minus bettas) and only stock like 4 max (obviously only one bettas though).
Honestly, for a 2.5g, I'd stick to a beautiful betta. They are super easy to care for and will be much more forgiving in 2.5g, just find s healthy looking one in the store and you should be good to go. They will also love to lounge around and in your plants.