That is adorable, but since some of the activities require planning (at least for me) like a trip to the beach, the zoo, etc...I'd put the activities on the front and just tell the kids that only one activity per week can be away from home.
It would also be a useful thing to have a range of ideas for the holidays when the kids are bored - especially when there are lots of things to be done. Like "you all have to clean the toys off the floor so grandparents won't fall over them, and then you get to pick one item off the washing line of stockings and candy canes" in the Christmas break. Am I bold to think of that as bribery?!