Using body weight and leverage is possible to put stresses on the body that would correspond to a couple of hundred pounds in the gym. For strength, there is the one-armed push up and the one-legged squat as described in The Naked Warrior. The best thing about these exercises is that they can be done anywhere. Conventional gym wisdom teaches you to do a set to exhaustion. This is not a good idea with these. Instead, do a set that does not exhaust you every hour on the hour. Alternatively, do it whenever you go into a specific room, like the bathroom or the kitchen. 5 push ups and 5 squats. Another exercise is the handstand pushup. Do a hand stand against a wall and lower yourself down in a controlled fashion. It should go without saying, but I will say it anyway. Know your limits. If you can’t press at least half your body weight in the gym, the handstand pushup is a dangerous proposition as it is much harder than merely pressing a dumbbell or a lever upwards. The handstand pushup is described in Combat Conditioning along with the hindu squat and the wrestler’s bridge. Hindu squats along with regular squats are power exercises. The amount of reps should be built up to the hundreds. Squatting for centuries replaces cycling, skating, sprinting, etc. for leg power.
The bridge and the one armed push up will develop everything you need in the mid section but if you are not strong enough for these, just do regular sit ups. Similarly, if you can not do the one-armed pushup, start with two armed pushups.
Finally in the world of free exercises we have to consider cardiovascular exercises. I suggest the burpee (see youtube video). Sets of these is fast succession will work the heart and the vascular harder than running because blood needs to be supplied to the entire body, not just the legs. If you do these on a regular basis, you do not need to run.
Now I could design a few programs with these exercises, but usually people are very very different in their capabilities. What seems impossible to one person feels like a cake walk to the next person, because the strong (wo)men are about three times stronger/faster/endurant than the average wo(man) and similarly the average (wo)men is three or more times stronger/faster/endurant than weak and out of shape (wo)men. This is a huge gap to account for.
In any case, you have to start where you are. If you know how to do one-armed pushups, do those. If you don’t, learn how to do them. If you can’t, do normal pushups. If you can’t, do girlie pushups. If those are out of range as well, do planks. If you don’t know what a plank is, find it. See what I mean? Once you find you level and your limits, start pushing those limits. Those who fail are those who lack discipline and consistency. Not those who lack money, equipment, or access to a gym or a trainer.
Working on personal fitness is almost like building capital by saving money. The more you have, the healthier you get. The healthier you are, the less you will be sick. The stronger you look, the more attractive you will look, and the easier things will get with other people – it’s a self-confidence thing. The stronger you are, the less limited you will be when you get older. I can’t even imagine worrying about bone strength when I reach 70. As with finances, there easy, pop a pill, get there fast methods, are generally inferior to discipline, making a choice and sticking to it. Decide to get in shape, physically and financially, and just do it.