Usually some instrumental post-rock. Often it's soft enough to relax you, but will have certain crescendo sections and quirky bits to keep you entertained still. e.g. Mogwai, Explosions In The Sky, If These Trees Could Talk, 65daysofstatic, God Is An Astronaut, Sigur Ros, Yndi Halda, This Will Destroy You, Godspeed You! Black Emperor .etc.
Acoustic stuff works quite well too. Andy McKee, Antoine Dufour, Trace Bundy, Eric Mongrain .etc.
Generally nothing with words, because I just end up singing along or typing out lyrics from the songs accidently. :P