Playing the guitar/musical special interest... Though I guess that doesn't relax me as such, just occupies my mind which is still highly desirable 
Meditation, which I recently got back into thanks to a post on here. Yoga as well, though the class I go to is incredibly intense. By the time I get through the 45 minutes of crazy 'MAKE YOUR LEG TOUCH YOUR FOREHEAD! AGAIN! NOW FROM THE BACK! STOP! PRETZEL TIME!' and into the relaxation part of the session I'm usually feeling pretty loose, which is great.
Listening to Sigur Ros is incredibly relaxing, though sometimes I have really strong emotional reactions to it, so I have to evaluate whether I'm in a good space to listen to them... Testament to how powerful their music is!
I recently bought a foam roller, which is incredible. Taking the time to spend 5-10 minutes rolling my shoulders and legs out is great. I bought a little spiky massage ball yesterday, which you use on various muscle groups. I used it on my lower back, which is often sore, and started to feel a little spaced out... By the time I was done I felt like I was floating slightly and crashed out for 3 hours. Love it.
In fact, given that muscle tension is often an issue for us Aspies, I would highly recommend looking into self-massage options such as those! I think it's going to take a while to learn to use the ball properly, but it's obviously of great benefit to me and I'm definitely making it a part of my daily routine.
EDIT: Forgot to mention, forests! Beautiful quiet places. I'm looking forward to moving to the hills in a couple of months, I hope it will help with my overall anxiety levels to have my home base in such a beautiful spot.
_________________
How did I get here tonight? What am I doing here?
How did I reach this state? How did I lose my sight?
I'm lost! I'm freaking! And everybody knows!
Everyone's watching!
So here... Are my hopes and aspirations
Nothing but puke
God, I'm so loooooonelaaaaaaayyyy
*power stance, air guitar*