Awhile ago I saw a pin about turning a pack and play into a toddler bed. (edited to add: here's the original link, which I'd pinned but for some reason my pin made the link all wonky)
One day I was having a rough day with the babies, and they kept wanting to sit under the table at my feet while I worked, and I remembered the pin. I had a pack and play in the closet that we haven't touched in a year, and so I was all 'OOH TWO MINUTE CRAFT!'.
I set it up, and then cut the mesh out of one side.
I left the other three sides intact, since Liv tends to fall over and Maddie tends to just throw herself into things. I felt like if the sides were open, I was just asking for frustration.
I put a regular-sized crib sheet on top to make a ceiling.
And then I folded a blanket to keep on top of that in case they want to play with the tent 'closed'.
I let the blanket hang down over the front, and they think it's their own little house.
It took me about five minutes to put the whole thing together, and they play in it daily. It's more sturdy than a play tent, so it doesn't fall over or get smushed. It doesn't move, really.
That may have been the best five minutes I've spent all year.
A couple of pointers.
If you have the mobile, it would keep the crib sheet from hanging down. I couldn't find ours, but the babies don't care.
I put a small blanket under the mattress to hang down over the front, because they kept trying to put things under there.
When you're cutting the mesh, you ONLY cut the white part. If you cut the sides (the brown on mine), you're going to mess with the stability and it'll start to fray.