To piggyback with my home gym thread, I would like take a minute to talk about what you can do if you are a poor college kid, live in an apartment and so on. This is the philosophy that I've been referring to as "The trunk gym" as in "will fit in the trunk of your car, is always available and can get you results equalling those of any commercial gym."

A small anecdote to start us off.
This past weekend travelled up to Chicago for work. Because I typically hit a strongman workout on Saturday, and accomplish all of my gym work through the week, there was no reason to try to do another gym workout. So I loaded up some of my training gear in my car and hauled it up to Chicago. Here is what I took:
A tire modified for dragging (IE I drove a U bolt through the tread), three kettle bells (25lbs, 45lbs, 70lbs), a 6 foot length of rope, a harness, a sandbag, and a box that I built for plyo work. Nice and simple.

So I hit the gym briefly with a couple guys from work, then "lured" them out to the green space outside our office with a sly "hey guys, come check this out."

What followed was absolutely eye opening for the two (and then eventually six, including two women) that I worked with. I set up a simple sled drag using the tire. 100ft walks, relatively minimal weight. All involved were breathing heavy, legs were burning, etc. Same set up, but pulling backwards this time. More legs burning, heavy breathing, etc. On to some kettle bell swings. This was probably the toughest thing for them to get used to, that little hip pop that is so important. Onto some sled drags, but this time without the harness, holding onto a rope. We talked about row drags, press drags, etc. Onto some box jumps, then sangbag clean and presses. At this point everyone else was taxed and they left. I did more sled work, KB clean and presses and more sandbag clean and presses. What part of my body didn't I hit?

While I have some expensive pieces in this scenario (the KB's are costly) there is no reason that you can't put together a low tech equivalent.

If I were a travelling salesman...
If absolutely everything I needed to work out with had to fit in my car, what would I take? Ten items, the ten most important in my estimate.
1) Length of rope. This can be fashioned into a harness, can be used for drags, dips, pullups, a "finish line", push ups, rows, and probably a dozen other things I'm forgetting. Thicker rope will hit your grip more, but will also limit the utility. I would stay with something in the 1" range, though 1.5" isn't too bad for bigger guys with hands like bear paws.
2) A sled. This could be as high or low tech as you want. A piece of plywood with some hooks to attach said rope to, an old tire like I use, an old car hood... whatever. Be cheap, get ghetto. You are going to abuse this thing, so don't worry if it is ugly. Put a few dozen hours of pulling on it and see how good the expensive stuff looks

Aside from your normal human ox pull gig, sled work is perfect for all sorts of back and leg work. If you want something that will build knee strength and support, backwards sled drags are awesome.

3) Sandbags. This is going to be your new best friend. Anyone that has wrestled with a heavy sand bag will attest to just how diabolical these can be. The softer the pack, the worse it will be. Sand flows, resists your efforts to move it, and forces you to make all those little core stability shifts that build true strength through the midsection. Cleans, presses, bear hug squats, shouldering, shouldered squats, rows, SLDL, curls, tosses, snatching, pullovers, etc... it is a blob of weight. Push it, pull it, do what you wish. It is the resistance that matters, and the way you have to adapt to a "live" weight that doesn't want to be lifted. Get a few of these bad boys together and do sandbag shuttles, climb across the monkey bars at the park with a sandbag between your knees... etc. Get the idea?

4) Resistance bands Any of this work that is hard can be made that much more challenging by adding resistance bands to it. There is so much that you can accomplish with resistance bands that won't even do it the disservice of trying to describe the little bit I can think of. Look them up online.

5) 2" pipe Pvc will work, will have a little flex to it and it is slippery like a greased eel when your hands get sweaty. All the better. Metal fence post can work too. With this you can create a wicked yoke setup using your rope and two sandbags of equal weight. Run the rope through the pipe, tie the ends around the sand bags, bam... yoke walking! You can also use this same type of setup for zercher squats, normal squats, deadlifts (on a 2" pipe this will truly test your grip and build amazing hand strength. Get creative with it! Ever seen any drils that pole vaulters do to build vaulting strength? Wicked.

6) A box.
Make it yourself, pay a friend, "find one" ;) Whatever. Make sure it can handle you jumping on it, loading sand bags in it, squatting onto it with your sandbag squat rig, etc. Just make sure it is bomb proof. Step ups, box squats, depth jumps, a box to lay on for pullovers, etc.

7) A big rock
Look, don't overthink this one. It is big, it is heavy, you know that you will look utterly insane in the middle of a park picking up a stone bigger than a small child and sweating like a fat kid at the candy machine... but that is AWESOME! I started lifting stones about a year ago, and I swear to you that I hardly walk by a stone without thinking "I can lift that... I wonder if I can get that on my shoulder..." etc. The sheer brute nature of it is the reason it rocks (pun intended). If you can't find a rock for cheap... ;) Heck, don't even worry about dragging it around. Landscapers have graciously provided hundreds of thousands of little gyms all over the place. Can't lift it? Flip it. Can't flip it? get your dirty little fingers under the edge and wiggle it around.

8) A basketball...
Or a big playground ball. now stab a hole in it and fill it with sand. Weee, a medicine ball! Wrap that sucker in 20lbs of duct tape and throw it around like it stood you up for prom! Need help figuring out what to do with a medicine ball? Look it up. This subject has pretty much been beaten to death.

9) a jump rope
Because you need to be in shape and when you combine a little hill running and jump rope in a circuit you will wish that you had never met me... no really. It is hard, but hard is what gets results.

10) Stop watch.
You probably already have this, but I'm a gadget guy, so I like my options. I would go with something that has a countdown timer. This way you can set up "rounds" easily. Set the time, start on an exercise, alarm buzzes, move to the next exercise and start the timer again (one button push starts the same count down). An egg timer also works, but can't be set for 30 seconds or something that refined.

There... ten things I wish I had in the event that I was broke and owned a car.