Protien.

I am under doctor's orders to eat at least 50g protein every 3 hours, which is HARD to do, so I have a nice long list for ya.

Snacks: hard-boiled eggs (eat the whole darn thing, not just whites, or you'll miss out on half the nutrients!), almonds, cheese wedges or cheese cubes, beef jerky.
My George Foreman grill is my lifesaver: you can grill (or bake, if you don't have a grill): fish filets, shrimp kabobs (with mushrooms & peppers, mmm!), pork chops, chicken, or steak.
Some people use beans and/or tofu for protein, but I can't eat those as they have too many carbs. Don't know whether you're watching your carbs or not.
Some people already mentioned Greek yogurt: I usually get the plain kind (flavored kinds usually have too many sugars & carbs for me) but then you can add flavor using part of a Crystal Light packet or a scoop of flavored protein powder.
Which brings me to protein powders: they are my other lifesaver. I throw a scoop in a glass of water and drink that with every meal or snack. You can get it in flavors from chocolate truffle to lemon tea to cappuccino. Just watch the carb counts; some are a lot higher than others.
Good higher-protein, lower-carb veggies are spinach, mushrooms, asparagus.
I work at the Renaissance Faire, and when I'm there I live on turkey legs (of course!), scotch eggs, grilled chicken sandwiches (without the bun), and sausage on a stick.
I would NOT use any of those so-called "protein bars." Most of them are just well-marketed candy bars, they have a ton of sugars & carbs along with the protein. Even the ones that say "low-carb" have an awfully high definition of "low."

Hope this helps. Good luck to you!