Color by numbers are one of these go-to activities for me. Here's a freebie color by number and it even has a back-to-school theme! Kids solve the doubles math problems (all within 10) and then color the cute picture according to the answer they get. And while the kids are busy with that, you can find places to stash the 3,000 glue sticks the kids brought, or go hide under your desk! ; )
Have it handy for one of those hectic moments when you need everybody to just simmer down : ) You'll thank yourself.