spray paint measuring tape and keep track of kids heights at different ages. Takes up no space and can move with you. I would paint the back side to mark so you can still see their height without having to remeasure your measuring tape.
Use rubber band or duct tape to secure sock to the bottom of the water bottle. Add some color to the bubble snakes with food coloring. The kids must be very happy to see the rainbow bubble snake created by them in summer.