Personally, I'd suggest plants; I'm not a big fan of the 'I have a tank so I need to get something' mentality.
I'd avoid turtles, first; most species get *much* bigger than can be housed in such a small aquarium, and they require complex setups (filters, monitoring water quality, all that crap) to deal with the immense amounts of waste they generate. Cresties might work, if you can turn the tank on end to give them vertical room; that might also be good for some tree frogs.
As for balls, they're usually great *if* you get captive bred (if it costs less than $60, it's not captive bred, no matter what the shop says). Wild caught and captive hatched are invariable parasite-ridden, usually die, rarely tame as well, and go off their food for months. Captive bred balls will also go off food for months, but not like wild-caughts or captive-hatched.
Personally, I'd recommend some Neps.
Mokele