I don't have a great idea, but here's one to start with until someone gives you a better one... (yes, I admit it is very, very, very corny)
You would need a box, a calculator and a bunch of numbers. The box can be cardboard or, if you want it to be a bit nicer, get one of those chipboard boxes from Michaels or a similar craft store. You can leave the box plain or cover it. If you decide to cover it, see if you can find wrapping paper or fabric with a number theme (you might look at Michaels or Joanns if you have those in your area). For the numbers, you can get magnetic numbers (find them at a teacher supply store) or use wooden numbers (check Michaels or a similar craft store) which you can paint or leave plain. Fasten the calculator to the top of the box; fill the inside of the box with numbers and put your response on top of them: "You can count on my answer to be yes."
