A center activity that engages all students.
I bought key chains from Oriental Trading (http://www.orientaltrading.com/clear-key-chains-a2-13752901.fltr) and attached these addition sentence to the key chains. I then attached the number to the locks. Students have to solve the addition problem on the key chains and check their work with unlocking the lock.
Need locks? I got a bunch from the Dollar Tree and also asked parents to donate some.