KABOOM! is the best center activity EVER, according to Jillian Starr of Star Spangled Planner. It is quick and engaging, can accommodate any content area for as long as you need it to occupy kids, and the price tag for putting it together is minimal.
The magic is in popsicle sticks (the high-quality kind that will not cause splinters), and prepping the fun will take a little bit of time, but once you are up and running, KABOOM! will be your forever friend.