Whey protein is the first choice that comes to mind.
Myself, I put two scoops into a shaker (cup with a lid), then when it comes time to down it, just add water and shake. That's about 44 grams of complete protein in one drink.
If you don't want to go that route, you could pack cans of tuna. At approx 30 grams per can, it's about as protein dense as you'll get.