The thing I did with a swamp IMC, was to actually make it a demi-plane in the para-elemental plane of Ooze. Plus, once they walked into it, they couldn't leave. They had to find a portal out. The only thing that knew where the portal was was a hive-minded collective of snakes that had been dispersed by the local population and couldn't gain any intelligence because the locals kept eating them. So, once the players finally put about 50 snakes in the box, they could telepathically communicate with it, and it told them what they needed to do.
That might be a bit *out there* for your game, though. Still, it was fun.