I'm a fan of yoga for relaxing purposes, I dont do it personally but I do meditate for 20mins every morning to get me in the right frame of mind for the day.
I wouldnt go crazy with the yoga/stretching stuff though - studies show that gaining flexibility will decrease core strength.
Its physically impossible to build core strength & flexibility at the same time, its a one or the other type of deal so you need to prioritise based on your goals.