Thoughts.

1-arm Dumbbell Snatch: This is a good, pretty easy, explosive lift. The slideshow explains it far better than I ever could; just focus on exploding through your hips and you should be on the right track. You can do it for low reps and work on strength speed, or do it for higher reps and work on conditioning/explosiveness. Definitely a quality movement (see also the dumbbell clean and swing for similar variations; the barbell versions are tougher to learn and do, but are very worthwhile to learn)

Pull-ups: Yep, pull-ups (I’d hope I don’t need to explain how to do these). Vary your grip however you like, just work on pulling strength. Lots of people struggle just to do a couple pull-ups; doing 10-20 every time you’re in the gym, whether that be 5 sets of 4, or 2 sets of 10 on the assisted machine, whatever–you’ll eventually get stronger and better able to do the real thing (I can manage 10-15 at the moment, up from maybe 4-5 in June). So much of what’s typically done in the gym is pushing–pushups, various presses–pull-ups are a good way to work on the pulling motions for balance’s sake. Your rotator cuff will thank you someday (rows are good too, whether bent-over, on a rowing machine, whatever). Chin-ups (grab the bar with your palm facing you) in particular are good for the lats, which are involved in just about every upper-body motion you could think of. I haven’t tested my max in a while, but I’m definitely over a dozen dead-hang pull-ups at this point (I can do them with an extra 45 lbs for one rep, as well).

Deadlift: See here for the wikipedia entry if you’re unfamiliar with the lift and its benefits; a cursory search of anywhere will give you information how to deadlift, it’s one of the most fundamental lifts there are. Things that are important to emphasize: keep your chest out–think trying to puff up your chest to impress that cute gal walking by; the bar should be touching your shins when you first address it on the ground and before you lift; make an effort to keep the bar as close to your body as possible on the way up–this is the safest way to lift it, and you’ll get immediate feedback on your form because you’ll hit your knees if you don’t use your legs to get it up before your back comes into the equation; and stabilize your spine by pressurizing your stomach. Think of your spine like a mast of a ship, with your abdominal muscles the tow lines stabilizing it, pulling from every angle. Contrary to popular belief, sucking in is actually worse for your spine. Definitely don’t go too heavy too fast, and don’t deadlift heavy too often in a short timeframe (I deadlift maybe once-twice a week right now). A worthwhile variation is the Sumo Deadlift + High Pull; the first Crossfit workout I did involved SDHPs for something like 30 reps per round; I was using just the bar (45lbs), I think, and was just absolutely destroyed after three rounds. That was when I knew I was on the right track with Crossfit.
The deadlift is probably the lift most responsible for my increases in athleticism earlier this year. I went from having a fairly tough time with something like 75lbs in late spring to pulling 215 in early October. And there’s still plenty of room for improvement, too. The main thing with the deadlift is that it works your posterior chain–glutes, hamstrings, back–all of which play a big role in athleticism, particularly running, and which tend to be overlooked and under trained because they’re “out of sight” so to speak. If you haven’t been working this area, even a rudimentary bit of strength work will probably make a big difference. It’s very much worth learning.

Front Squats: My other favorite “big” lift. As opposed to the more commonly known and practiced back squat, where the weight rests on your shoulders, with the front squat you support the weight on your shoulders in front of you with your arms (very) loosely keeping the bar in place. I prefer the front to the back squat because it’s a lot harder to mess up your back with due to improper form, and because of the extra flexibility required to do a full-depth one. A good front squat requires pretty good arm/wrist flexibility, and I also think that because of where the weight is held one has to be on their heels. Too many people squat one their toes–aside from being hard to balance, it’s symptomatic of poor ankle flexibility and isn’t how you’ll be able to lift the most. Another thing that’s helped me a lot is the improved flexibility from doing things like full range-of-motion squats. Start light with this until you develop good flexibility to do it with heavier weight; focus on pushing through your heels, and extending at your hip. Hip extension is the name of the game, and it’s what’ll carry over with good effect to your day-to-day athletic pursuits. I’ve worked up from 135 to 175 in these in the span of a couple months; they’re pretty tough, but rewarding. If you really want to find out how inflexible you really are then try overhead squatting, with just the bar. There’s a fun exercise.

That about covers it. These are the first exercises that come to my mind if I go to the gym without a specific plan, because I like doing these exercises a lot.