This is an activity which extends the TERC Investigation "Peas and Carrots," which is a very nice investigation, but ends way too soon. So I upgraded it by turning it into a detective game: students take a stack of cards which use different number combinations of the same two items (11 beans and cornbreads, 10 platypodes and penguins, etc.) Students take the stack of cards, remove one from the deck, and then look at the other ones to figure out which one is missing.
Along the way, students learn to find opposite combinations, what happens when you have an even number of objects (there's always a combination without an "opposite") as well as subitizing small numbers of objects AND writing addition equations! How much fun is that?
Comes complete with combination of 6 - 12 cards, recording sheets, explicit and extensive instructions. You'll love it!