The elephant in the room is sex. Very relaxing afterward, no two ways about it.

Other methods have been listed also. Mine are in no particular order:

Laughter--what a great natural release. Right there after sex. :)

Playing a game with the kids

Playing the piano

Listening to music

Listening to a book on tape

Reading a book

Walking the dogs

Creating something--sewing, for example, or working with wood

Gardening--the one I probably do most, next to dog walking

A power nap. I am a firm believer in the health benefits of a short nap in the afternoon. 15 minutes or less, usually.

SRS