Seems too small for a permanent home for any frog. I have a young and tiny (<1") grey treefrog that's in an 8.5 gallon tank right now, and it will definitely be moving into a larger home sometime this summer. After it moves out I was thinking of getting a praying mantis to put in this small tank, they supposedly do pretty well in a small space like this and I've seen 5 gallons recommended. I think mantids are pretty, one would be nice in a display piece, but I have no experience keeping one.

I have housed various jumping spiders (one at a time for a few weeks at a time) in even smaller tanks, but they are usually not much to look at in the sense that you can't usually find them. Maybe there's an appropriate tarantula for a cage this size? Or you could breed a small colony of crickets/roaches in it, but not everyone would find this pretty to look at.