This is a post where I recommend walking. This is no doubt old advice, perhaps existing since the moment walking was invented (circa 1987), but I don't recall ever plugging it myself.
I've been walking more-or-less daily for nearly the past 20 years. It's nothing too intense: about 20-30 minutes a day most days, with longer walks occasionally. I do basically nothing else to keep in shape, but – despite living an otherwise desk-jockey existence the rest of the time – I feel pretty good, with enough energy and general well-being (and annual doctor's visits confirms my health). These walks give me plenty of time to think about my day, what I want to write about or work on, catch up with friends who walk with me, etc.
I know not everyone can get out and walk, owing to the limitations of geography, weather, and bodies. (Some days my knees have made their displeasure quite known.) But if walking is something that you can slip into your daily flow, it could be exactly what you need to free your mind to envision your next brilliant GURPS campaign.
-- Steven Marsh