Gyms. You join, you pay, you never go. And if only they were as good for the environment as they are for your quads. Pool heating, refrigerated drinks machines, vast arrays of TVs: how ironic that gyms are so energy-hungry, when gym-goers waste so much running on the spot and riding bicycles that stand still. And the only thing that's recycled is the air.
Makes you wonder what we did for exercise before our lifestyles tied us to an office desk and expanded our waistlines.
Some of the answer to that might lie in green gyms. First set up in Oxfordshire in 1998 under the wing of the British Trust for Conservation Volunteers (BTCV), these are greener, cheaper alternatives to traditional health clubs. Members "work out" outdoors, in the countryside or other open spaces, while all that energy goes to some use in conservation activities, such as planting trees, dry stone walling or creating school nature areas.
The exercise is moderately strenuous: about the right pace - research shows that people who regularly exercise in this way are 50% less likely to suffer from a heart attack or stroke. The BTCV's own research shows that almost a third more calories can be burnt in an hour in some green gym sessions than in a step aerobics class.
You exercise at your own pace, which is ideal for older people, cutting the risk of illnesses like osteoporosis, says the BTCV. "It also improves muscle strength which is particularly important for older people, helping to maintain their independence in later life."
There are other health benefits too. "We all notice the feeling of well-being that comes from exercise," says the BTCV. "This is enhanced by the fresh air and natural surroundings. And there's a feeling of achievement when the exercise has a visible result. It's much more enjoyable than exercise in a conventional gym."
Green gyms are also known to reduce stress: participants interviewed for the BTCV's research say green gyms have been good for their mental health, boosting confidence through learning new skills.
The Mental Health Foundation agrees. It says green gyms can provide a sense of accomplishment participants might not get in a traditional gym. "It has also been mooted that people have a biological attraction to nature - "biophilia" - and that connecting to nature can itself provide mental health benefits," it adds.
Gardening, getting muddy and being at the behest of the elements might not appeal to everyone, but of course, you can choose when you go. Anyone can take part, and children are welcome. There are groups around the country running activities throughout the week. Each session lasts for up to three hours. You'll do some basic warm-up exercises first; then begin with gentle tasks, building up to more strenuous activities according to level of fitness.
No special experience or kit is required: some old clothes will do. And best of all - unlike your local gym - it's free.
To join a group, or even start your own, visit btcv.org.uk or greengym.org.uk.