I totally recommend a mystery snail! I have one in my 5g with my betta, Cato and they got along much better than I thought.
Cato is usually pretty territorial, but turned out to be afraid of Sheldon for the first couple of weeks and now rarely pays any mind to him except for the occasional flaring.
Sheldon the snail is also really fun to watch sometimes- he's very active. :)
As far as cleaning goes, I have a turkey baster that I use to siphon out all of the snail poop that I can find every day. Pretty easy upkeep.
I've also heard that Nerite snails are great too. It's just a matter of finding them. You can get them online, but they cost $30 to ship. PetsMart is the only pet store around me, and they only sell mystery snails- which is how I ended up with one. Great choice though!