I actually think it's going to work out all right. Abby spends her time riding her bike up and down the sidewalk, and Makenna just chases after her. She may spend much of the summer showing the neighbors her cast all day... it'll get pretty hot with another layer over her, but that's managable.
They play with bubbles. Makenna isn't very good at it and makes a gigantic mess of the whole project, but that's part of the fun. And it's not enough moisture to hurt the cast.
While I was talking to a neighbor the other day, Abby was watering the plants with a little watering can. She ran out of water and asked if she could go inside and fill it up. She was inside with Makenna forever. When she came back out she was carrying to cups from the 'sippy cup' drawer. She said "It'll just be easier this way. Makenna and I filled all the cups and now we are going to bring them out here". Great. How much of a mess had they made? How wet was Makenna? To my surprise, the kitchen counter, kitchen floor and Makenna were all dry! So they spent the next five minutes carrying cups outside and the following 10 minutes pouring them down the driveway and running through the 'river'. And still no water anywhere but her feet :)
When all the cups were empty and they were asking to go fill them up again, I made a brave decision. I pulled out the hose. After we filled up the cups, I attached the sprayer and sprayed it way up in the air, letting just drops come down on the Abby and Makenna. They loved it! They laughed and ran and played in the rain we had made. And although her shirt was wet, the cast was just fine. After all, she's been out in the rain before; it wasn't much different.
And then I sprayed the hose accross the driveway and they ran in circles and jumped the water and splashed their feet in the small puddles. Still only water up the her knees; it was fine. We are gong to get through this just fine. She'll splash around with her sister and laugh and play in the sunshine just like last year. A little cast isn't going to stop us from having a great summer!