If I don't put them down together (they're 2 and 1, so they often nap together), then Adam does:
-lacing cards (shaped like tools! he "fixes" things around the house after)
-M&D pizza and sandwich boxes
-sorts manipulatives by color at the table
-sits at the table to "write" with pens/pencils, etc.
...that's mostly it.
Anything he does during his "quiet time" while I'm putting Sarah down has to be a mom-approved quiet activity. He's on a bed or the couch to do any of these things and has to stay there until I come back from Sarah. None of these activities (except reading!) are available without my assistance (they're on top shelves) -- so to get to do them....well, he's never not stayed in one place to do them while I'm with Sarah!
But he is pnly two - that is very different than an older child who can be trusted to safely use different toys/activities when you are not physically present on the same floor of the house.