Second the mantis. If you like carnivorous bugs (and have a lid on the tank) there are quite a few spiders, scorpions, etc that would do nicely in a setup like that. I think jumping spiders have a lot of personality, and a tank that size would be small enough that they don't just get lost but large enough to view their jumping behavior. You could keep several striped bark scorpions in a cage that size. A pair of beetles would work, and be a little more active, though they may eat your plants. The downside is that most of these animals don't live very long; usually only a year or two (though the scorpions can live ~5 years, and might breed for you, if that's something you would find appealing instead of horrifying).