Anything tangential to the problem you're working on. Ping-pong, hiking, walks on the beach, swimming, to name a few. I also found REM sleep or deep sleep to be

Anything tangential to the problem you're working on. Ping-pong, hiking, walks on the beach, swimming, to name a few. I also found REM sleep or deep sleep to be very reparative. Maybe not 'problem solving' but it sets the stage for the rest of the day, where you need a clear head and no brain fog.