I actually really like that plan. Of course, it wouldn't work in my hometown suburbia as well. Good thing I go to university in a cowtown. We've also got construction around a dam and a spillway, so rocks are never in short supply.
I bring them in to work out with me. Won't work with large groups, but I'm not a personal trainer by career, and I have classes. The nice thing about doing this is that you can oversee what they do, and get them to see what you do. Doesn't always work with the women scared of free weights. But the best feeling of my training life was getting asked by a friend of mine who was in marching band if he could workout. He said since marching season was over he was losing his abs. He weighs all of 150 soaking wet. Had never benched 135 before. Well, in a month he'd done that, and now two years later he just recently squatted 350 at around 160, and he's hooked. We had a mutual friend who asked to workout with us after about 6 weeks or so, now this kid lost around 40 pounds and has squatted 375. I was worried that first summer when I left them alone, but they managed to keep up with it.
Keys: you don't "work out", ever. Even though I use that phrase on T-Nation. You train. It implies a certain direction or natural development. Try to find something they're interested in and get them to internalize this goal. Then you explain how everything that they're doing helps them achieve this goal. You have to give them a reason so that they know WHY they do these things.
You have to be honest when you don't know something, and you have to be able to convince them that you went through the pain that they are currently feeling.
It basically boils down to making them feel like they're pushing you or contributing to your growth in some way. They feel useful, and at the same time they know you've been where they are. They may sometimes start to feel that they'll let you down if they don't show up (you "need" them). One of the primary goals is to transfer that "need" from you to them.
Make it clear at the very start that once we are in the gym we don't talk unless it's necessary, and we don't mess around. The gym is a sacred place to me, and it should be instilled in them as well (that'll take time). It also helps to start subtley implying that the majority of trainees are lost and mindless (they are). Just don't carry it to the conclusion that you are the only one who knows how to train. You find someone who's usually in the gym and personifies their goals and you point out the things that they do right. You also point out the things they do wrong, but only if asked about something your client sees them doing.
Above all encourage them and share their accomplishments.
I'm no personal trainer, but I have had many different situations where I've needed to motivate someone both in the gym and in other endeavors, and these things are what have helped me.