A friend of mine has a not-quite 2 year old in a spica. It does not look like fun! That said, she's crazy creative and they've been doing all sorts of things with him, including creating ramp-like lying seat thing so he can lie on his belly, but have his torso be up high enough that he can play comfortably with toys on the floor. They use a bean bag chair too. They've also used a little platform on wheels (like a mechanic would use under a car sort of idea). After a few weeks though, he's actually standing and almost walking in it, which is apparently very common.
I have no first-hand experience, but there seems to be a fair bit out there about spicas and keeping kids from dying of boredom while stuck in them. Sorry your kiddo is having this experience!