Or is there already one running?
I have a lovely fabric calendar which I put different things in for my 3 kids each year. I always seem to run out of ideas though.
This is what has worked well before:
Mini bubble wands
Chocolate coin
An 'experience' ( I only do this when it's pre booked anyway so it's not an extra expense)
Baking activity
Hot wheels car
Would anyone like to share ideas?