As someone who has lived rather isolated for long stretches, I think the one thing that helps fight feeling isolated is to find your joy and practice it. My joy is in creative outlets. When I'm alone, I sing, woodwork, cook new recipes, garden [indoors or out], sew. If I had a large indoor space I'd probably practice slacklining, aerial arts, tumbling, parkour, climbing. And then there's the computer, where I can create music and endless programs, websites. I haven't even touched on painting, drawing, playing music. And all that can be supplemented by podcasts, music, movies/TV, reading, chores, working out. There's really so much to do indoors if you can cultivate a creative mindset.