Indoor putting practice drills do not always need to be about stroking a ball into a drink glass. Your putting skills would be well served to add some exercises that hone your perceptual skills and specific stroke movements.
During the off-season, in fact all throughout the year, many golfers want exercises or drills to refine or sustain their putting skills. Obviously, you are pretty limited indoors by space limitations and also by the nature of a typical indoors surface (carpet, hardwood, stone, i.e. no breaks).
Still, if you focus on perceptual and movement skills that are essential to golf, you can get a lot of valuable practice. The point is to focus on developing specific aspects of the art of putting.