I was out yesterday volunteering with the local wildlife trust helping to clear a footpath, which had become overgrown. It's part of a local nature reserve that I started helping out on at the beginning of the year. It covers a pretty large area and there were only seven of us out working yesterday.
Much of the English countryside has been farmed for many years and so it isn't necessarily in it's natural state. Grazing of sheep, cattle and ponies has an impact on the variety of plants and animals that inhabit a particular area. Grazing here on the South Downs has resulted in the chalkland fauna and flora that exists today, much of which is rare and would disappear if it wasn't for the grazing and the work of volunteers keeping the scrub down.
Most local councils and wildlife trusts do not have the funding and resources to manage the land, so they rely on the work of volunteers. There is always far too much work to be done and not enough volunteers to do it.
If you'd like to get involved, it only takes up one day a month and most working parties don't work over the summer. You'll get out into the fresh air for a few hours. Work hard, there are always a variety of tasks for different physical strength and abilities. It's very rewarding even if you only feel your scratching the surface of what needs doing. Most days start at 10.00am and finish around 3.30pm although you don't have to stay all day.
If you work for a company - why not organise a day out for your team. Most companies these days get involved in the community as part of Corporate Social Responsibility so why not do something worthwhile and make a difference to your local countryside.
OK ... I'm sorry this is very UK centric but I hope that anyone in Europe or further afield can find something similar in their area. I found this for the US - National Wildlife Federation
If you know of any other useful links feel free to add a comment to this thread.