Sit ups, push ups, leg lifts, running in place, walking - can be done inside. Tons of strengthening and stretching can be done inside. If you have extra $$, equipment can be bought to use inside. I guess I am missing part of the question? Do you mean stuff for free inside, or just in general?
I live in WA state, so I have to have options to do exercise inside or else I'd lose half my exercise days due to weather.