One of my personal favorite pitching drills to do inside is pitching with socks. I learned this from Bill Hillhouse.
Find a mirror or any other reflective surface. A glass door will do too. Roll up a pair of socks and practice your pitching by pitching the socks right at the mirror.
In my house, I happen to have a full-length mirror at the end of a hallway. The hallway floor is made of ceramic tile. Not only do the tiles serve as a method for measuring how far I am from the mirror, but it also provides a built-in power line.
If you don’t have a tiled floor, you can find other ways of laying down a power line to practice with. A piece of tape works well. If you’re lucky, you may even be allowed to put some tape on the mirror to create different pitching zone targets to pitch to.