This happened to us when our kid was three. Is it a Spica cast? The toughest part is figuring out how to use the bathroom. That was very difficult for the first few days but we eventually found a solution that could work for our kid and keep the cast clean.
As for activities: wagon, yes. Walking around the neighborhood. Art projects, building things on the bed on tummy, science experiments, reading, some TV. A family member got us some caterpillars from a science site and watched them turn into butterflies. We ordered geodes to crack open, etc. If you can stand it, I’d order some “sit still” toys that are new and special for this time. It is a very difficult cast to be in and your kid deserves a little TLC.
Sleeping: our orthopedic surgeon recommended putting a mattress on the ground and having one parent sleep with the child. On the ground in case they fall off the bed. And parent with them because they can’t get up and help themselves if something is wrong.
Sitting: Prop up with pillows on the couch, grab a bean bag chair, prop up in bed, etc.
It was a long haul but everything turned out ok and our child was strangely very patient with the whole process and idea.
Oh, and you might want to buy some books about casts, getting casts, etc. The removal process is a little upsetting for some kids (loud saw involved) and so that may help plant the seed of how the thing comes off.