Re: Indoor practice
From October to April, I cast with a club in a school gym. Accuracy is the main focus.
We use the standard ACA size targets that are about this size of a hoola hoop.
You can also purchase small orange cones (like for kids soccer, etc.) to use as targets or to represent rocks for practicing your curve casts. I like the frizbee idea a lot too, they would be easy to move around.
In the Dry Fly game you have all the false casts you want before the lay-down. In the Wet Fly game you have only one false cast before the lay-down (except on the first target).
On different nights we require casters to hit the targets in a different order - this improves line handling.
We also teach newbies and help more experienced casters with the double haul and reach casts, etc..
Use a piece of yarn for your fly.
Gyms floors are dusty and WILL DESTROY your line. You wil need to clean it after every use, but that still won't save it. As mentioned above, buy a cheap line for the purpose, or use an old one that you will never use for fishing again.