It was raining by our house the other day and our youngest (2.5 yrs) wanted to play baseball. Of course playing baseball in the house isn’t the brightest idea in the world… however we figured out a way to make it work. We used a soft baseball bat and a large blown up balloon. Since we have a large open spot in our basement, we decided to try it. We let “pitched” the balloon to our 2 yr old and she would swing away. Since balloons move slower, she was able to hit a couple times before the balloon would hit the ground. It helped to work on the hand/ eye coordination and she thought it was pretty cool that we played baseball even though it was raining. Just be sure that you have enough room to adequately do this without hitting other siblings during the swing!
You could also devise a decent game of indoor golf with your bat and a hacky-sack or tennis ball.