Well, he doesn't know how to open doors yet, so that's a big help. But, it's not like he spends time at the door trying to open it or asking to come out either (which he will do if he wants to be done).
We allow him board books (the demise of one of his favorite paper storybooks prompted a "board books only" rule) and then the stuffed animals on his bed. He does a lot of "reading," talking to/pretending with his animals, making glo-worm play songs. We have a video monitor so we can periodically check on him. He also likes to lay on his back on the bed and kick the wall, and occasionally be reminded that quiet time means "quiet," but usually one verbal reminder and a cup of water is all that's necessary.
Once he can open doors, I imagine it will be a whole new adventure. I plan to do a little bit of enforcing - "You can either take a nap or play quietly, but you need to stay here for a while" - but we'll see how he does. OTOH, he really seems to enjoy the time and I think that a little time by himself is a good thing, so maybe it won't be much of a fight. I can always hope.