You could get them to make spinners (cereal boxes seem to be the best card, ime), either the sort that is a regular polygon with a toothpick through the middle (easiest) or the round sort with a pointer and a split pin (will need a trip to Ryman!) (and fiddly to make because they will probably need your individual attention to make sure the pointers spin smoothly). They can predict how many times it should be green (or whatever), and then spin it lots of times to see how many times it is green. While they are doing that you will be able to circulate and be observed asking good questions. Or the horse race game is always fun. Really it's not about the activity, it's about how you deliver it and whether you tick enough boxes (VAK, feedback/AfL, high expectations, demonstrable learning in every child, good use of time, real-world connections, keep on top of behaviour, differentiation, a good review / plenary; is a good shopping list...) Good luck!