Take a look at this submission from Wes. A creative idea for softball throwing drills to use when forced inside to practice. This is one that can work with all age groups.
I have a softball drill I use indoors when we can’t get outside. It is designed to help my first basemen learn to squeeze and give when catching the throw to first base.
We use tennis balls. A softball player (usually my second basemen or shortstop) receives a bounced ball from a teammate, simulates a force out at second, then throws it the length of the gym to the first baseman. The first baseman steps to the ball and catches it with her bare hand. She must squeeze the ball as she catches it and give with the catch or the ball with bounce off her hand.
This helps the first baseman and the middle infielders who are working on their footwork and how to throw around second base.