I agree with Moose, sex works really well for relaxation!
Other than that, I'm notorious for
not relaxing. Hiking is one of the only things that always works. Going backpacking is like hitting a reset button in my brain - all I have to worry about is survival, putting one foot in front of the other, not getting lost. Life is simplified to the essentials.
I do lots of other things to keep my hands and brains busy, but it's not exactly relaxing. My hubby makes fun of me because my idea of relaxing is his idea of working.