I've just become a signed up member of the South Downs Volunteer Rangers, having just spent my 'taster' day out on the Downs. I knew I'd enjoy it, I've been volunteering off and on for a while and more recently with the Sussex Wildlife Trust at Lewes.
It was the best fun I've had in ages. A full-on day, blooming hard work and I arrived back home very tired, covered in scratches and smelling of smoke but it's that good physical exhaustion, not the mental exhaustion you get from spending all day in front of the computer and certainly a great stress-reliever. Fresh air, loads of exercise and a sense of a job well done.
The South Downs Joint Committee is one of several groups that have working parties going out three days a week and also at the weekends. Most companies are willing and able to get more involved in the community, through various Business in the Community schemes.
Why not see if your company is willing to get it's employees involved. Arrange a team day, out of the office, get involved in your community. If you want to do something energetic, then go scrub-bashing on the Downs.
You'll ache but you'll feel a whole lot better for it.