Use Your Body (or someone else's)
This is where a buddy is great. Have them provide the resistance by grabbing your forearms. It may take some finagling, but you'll get a good position. You can do this for just about any free-weight exercise. Curls? Done. Bench press? Done. Flys? Done, done, done. The great thing is, with a little bit of adjustment, your partner can turn it into an exercise for themselves too, especially if they're a little weaker than you.
But Jim, I'm too sexy for my shirt, much less friends
I know, I understand. If your friends are all intimidated by your dead-sexy Apolline physique, you could use an exercise strap around your thigh instead. Or around a fence post, lamp post, huge freaking rock, whatever's not going to break on you.
You mentioned Gravity, I loved that movie!
Great, I can work with that! If you're stuck alone, you could also use this movie as inspiration for a workout: Try just working against gravity. For example, for triceps: Lift your arms over your head, like doing a triceps stretch... But instead of stretch, do hammer curls--I'm using the term loosely, but Ryan points out in the comments this is called "Dumbbell Tricep Extension". Cool! This obviously does not work your chest all that much.
For chest, you can lay on your back, reach your arms out wide, and lift them up an inch off the ground. And then... well, you could just hold them there. I'm not even kidding, just hold them up to where every inch of flesh on your arms is one inch off the ground. Turn on Gravity. When you can watch the whole movie without lifting or dropping, you will have... ... well, probably really weird looking pecs, if we're honest. You might try working just a narrow range of motion. Such as--
I like Michael Bay instead
--Cool, wow, that's an apropos interjection. Let's adapt that Gravity pec workout: Lift your arms one inch off the ground, and move them through, say, 30 degrees of motion; the arc is the same as if you were going to bring both palms together in front of your chest, just like doing fly's... but only do 30 degrees (or 16 degrees, or 6 inches, or whatever inspires you). Once you get the hang of that, move your arms up against gravity with explosive speed--but when you bring them back down, bring them down slowly. We're talking, say, 10 seconds to drop back down through those 30 degrees (or 16 degrees, or 6 inches, or whatever). DO NOT let your arms touch the ground.
This stuff's awesome, but I got inspired to work out watching Road Warrior
If you want a post-apocalyptic theme to your workout, jump right in! Do high reps with your rifle (unloaded, properly cleared, etc) as a load for, say, bench press, or reverse curls, or I don't care, again anything you'd use free weights for. A rifle's going to be somewhere between 5-10 pounds. Or, if you're stuck in California, accept my apologies and use your day pack instead (or just your medical pouch, depending on how you pack). Or just a gallon of water, around 8.34 pounds. Here's the PDF of Army FM 21-20, Physical Fitness Training. There's a lot of good stuff in here, I like Rifle Drills, found on page 8-11. Perusing this also reminded me that exercise in just water provides some pretty good resistance too.
What about laywers?
Do you mean the ones who will sue? OK, here's an obligatory: I'm no physical fitness instructor, I've just led military PT for injured people. With no training. I was assigned the duty because I was also injured and recovering, and had the common sense God gave a brick... not because I'm trained. So stop if something hurts. Talk to a real trainer before starting this stuff. etc etc. If you don't, then you're taking your health into your own hands. Obviously.
OR do you mean the lawyers in the gym who say you can only exercise in lycra, with steel bars, using machines with pulleys and plates, and only in the ways people have decided are normal? 'cause screw those guys. You came to a website for advice, so you're probably ready to consider the unorthodox.