I don't think it's the least little bit wrong to say that only limited amounts of stuff be out at once. I don't think it's necessary to require everyone to participate in the "stuff" acitvity--they just don't get another "stuff" activity.
For example, right now we're playing with crayons and coloring notebooks because it's Joe's turn and he chose, and if you don't want to do that you can get out exactly one book, or you can go run around in the yard with the dog, or you can do anything else doesn't involve getting out "stuff". And tomorrow (or this afternoon, or in two hours, or whatever) it's your turn to choose.
And Joe, if you're bored with your choice and you want another one before your time is up, please put away all the bits for crayons and notebooks, and then come to me. Remember that you only have until 2:30, because at 2:30 everybody cleans up and gets out the stuff for homework.