When all else fails, just give up. It's a classic.
Find a comfortable position, like your couch or a hammock, and just... stop.
No need to stress about productivity. No need to think about deadlines or goals. Just let your brain melt into the void.
You can even make it more efficient by setting a timer for, say, 30 minutes. When it goes off, just reset it. Repeat. Rincedet.