Another rule of thumb is doing things that are manual in nature. Playing solitaire with real cards. Doing hand crafts. Basically, if you limit your stimulation to the speed that your hands can move, the brain should be able to remain relaxed. Playing a musical instrument would be OK. Learning card tricks. Drawing, sketching, etc. Avoid any activity that causes frustration.

Knitting or crocheting works too. NFL football players do both to relax.