For me, having nothing to do is a luxury. When I find myself in this mode, I take long walks, let my mind drift and think about whatever it feels like (usually it chooses to think about one of my ongoing projects, big unsolvable world-scale problems, future, lack of moral progress, or sex), read long-form stuff (mostly Kindle books on my phone) and generally relax and recharge, assuming that I can find a relatively quiet environment.