What are cool/unique invert freshwater pets?

My boyfriend's birthday is coming up.. he's REALLY big in animals, especially the small ones.. recently he's been big into aquariums, he already has a few..

I just got a 15 gallon tank hoping to fill some open space for him, I have several aquatic plants already, gravel, substrates, etc. (and can always buy more) however I have absolutely no idea what to put in there.. any suggestions?