Make a 'Counting and Adding Machine' using a cardboard box!
This is an idea for any number operation.
Cover a chair with a large piece of material and put a sign on the front describing the machine (e.g. "adding on 10" machine etc).
Then get 2 pieces of card and put the starting number on the first piece and the finishing number on the 2nd. Sellotape the top card to the bottom so that it can be used as a flip card.
Explain that the machine will solve problems for us and pass the card under the chair while the machine is 'working'. When under the chair flip over the card to show the finishing number.
This can lead to discussions about how the 'machine' solved the problem and what will happen to the next number.
The children love this and this can then lead to a human 'machine' with 1 child saying a number and then after a few machine-like moves the second child can solve the problem.
This works for any operation on any number. Great fun!