i think that activities are the best way to attain and maintain your health. instead of trying to figure out which muscle groups to work, how many and what weight, etc. just find some high intensity, fun activities and give yourself a real reason to exercise consistently.
personally, i find the gym to be BORING. if i wanted to work out upper body, i go rock climbing instead, for lower body, mtn climbing/hiking, running, riding a bike, ive even found ice hockey to wear your legs out pretty good. it will work the muscles in a balanced fashion since it's natural movement, like calisthenics, and it's something you can actually enjoy to boot.
Although I actually enjoy lifting weights, I like to do it not just in the gym but elsewhere as well. Engine blocks, tree trunks, cinder blocks, etc. can mix things up, and require different strategies to move.
For cardio work though, nothing bores me faster than being on a treadmill or elliptical machine at a gym. I would rather be out hiking or Fell Running. Anything but watching the woman with the 4-inch long fake nails, and a fake tan, try to start the treadmill without using her fingernails. :rolli: