Even if it's something simple like listening to music, getting a relaxing massage, or doing meditation.
I recommend taking a hot shower and using the massage jets to soothe any aches, sores, or pains.
I usually sleep when I'm tired. I take like a 2 hour power nap, and when I wake up I feel better than ever.
When I am having a stressful day, I always relax in a hot bubble bath in order to clear my head. The heat of the water and frangrence of the bubble bath is so relaxing. Sometime I even turn the light off and light candles, which makes it more relaxing.