Lime! We had those! Oooooh the memories. They were ace.
I was coming in to suggest a hammock/fishing net thing too... you could even theme the room a bit under-sea-ish if they're into that, hang fish from the bottom of the net, or turn the net into a boat that all the soft toys live in.

I want one when we move house, I reckon my kids would love throwing their toys up there at cleanup time. And conveniently out of reach too.