My advice here is go LOW LOW LOW LOW TECH.
I'm not a cichlid fan so these other folks in here are the ones to listen to there, but I would go with:
A. Puffers but keep in mind the small ones common at LFS's require brackish water despite what the wayward part time highschool employee tells you lol.
B. a school if smaller tetras like rummynoses.
C. Small spiny eels. peacock eels are the ones I find most commonly here, but any of the species EXCEPT fire eels or tyretrack eels would work in your 40, and they are personable little guys, most will eat from your hand.
Your options are mildly limited with a 40 but again all the suggestions I've seen are good.