We had one of those days.
And this is what we did: balloon tennis.
It couldn't be simpler. All you need is a latex balloon and tape as well as a disposable paper plate and jumbo wooden craft stick for each player.
Simply tape the craft stick to the paper plate to turn it into a paddle.
Blow up the balloon and if you want, use painters tape to make a line on the floor. Kids stand with the paddle on either side, lobbying the balloon back and forth, trying to keep it from touching the ground.
This may well be the cheapest gross motor fun my two boys have had together!
This idea came from Keeping it Simple.