My favorite activity to de-stress is blues dancing. Another favorite is birdwatching.
Blues dancing is awesome because it's exercise and it can get intense, but the motions can be slow and smooth, and even though it uses muscles all over your body, most of your body can be very relaxed when you're dancing. It's also very social and involves a close (and relaxing) connection with your partner. Especially when you do it with close friends or people you're very comfy with, it can be one of the most relaxing things in the world.
Birdwatching is relaxing for a very different reason...it is a slightly active activity, but requires minimal physical exertion--but extreme focus, paying attention to sensations (vision, hearing). In this respect it's oddly like meditation or even prayer. It's impossible to worry or think about anything else when you're focusing on watching a tiny songbird flitting around in a tree or trying to identify a distant hawk, sandpiper, or duck through a spotting scope. Being out in nature, whether in a forest or grassland or wetlands, is relaxing on its own...and paying attention to the birds helps you become connected to the ecology in a more intimate way.