Juveniles newts could go in there. For example, I have a T. marmoratus juvenile that could fit in a smaller space, but only until it grows larger.
I've also heard of people keeping Eurycea bislineata or other smaller Eurycea species in a 2.5 gallon, but I personally advise against it. A 5 gallon is small enough, but a 2.5 really isn't enough space for them to move around.
Furthermore, whatever you put in there must be terrestrial, unless you are rearing larvae. Water quality in a 2.5 would be difficult to maintain.
If you are interested in other types of tank critters, maybe try an invertebrate? 2.5 gal tanks are great for smaller inverts like beetles, isopods or millipedes. Millipedes in particular are quite fascinating and come in many pretty colors. Another idea could be a nano-tank aquascape, with some Neocardina / Cherry shrimp.
Good luck finding an inhabitant!