Not a fish, but a claw frog could work (it is illegal in your area). Pretty low maintenance and it’s fun to watch.
Paludariums are awesome.What about a palladarium? Could hold Red-bellied Toads along with some small Minnow-like Fish.
This is a 29 gallon.An Oscar would be lovely for a 1 fish tank..
I would agree with a puffer, but the op wanted something easy. Puffers can be a lot of work.