Well.... there's the obvious; add meat or seafood or egg or nuts to everything. This ranges from extra meat on sammies, to shish-ka-bob (you can make a bunch of small skewers ahead of time : teriyaki, indian, thai, buffalo, asada, etc., and keep a few in the fridge and the rest in the freezer all cooked and just needing to defrost... land creatures work best for the cook-freeze-defrost... but shrimp skewers are delicious and easy easy to make -hint, use 2 skewers so they stay in place), to eggs for breakfast / hard boiled in salads/ quiche/ etc. BTW... nuts are underutilized in everyday cooking. Roasted pecans (just take some pecans and put them in a dry frying pan and toast until they smell fantastic) are great in salads and in sandwiches, even sprinkled over pasta.
Less obvious would be the legume+grain (beans and rice, hummus & pita, any grain + any legume = a complete protein) and other tricks for getting proteins (nut milks, quinoa, etc.)
Also, there are protein powders. They come in GIANT tubs for weight lifters. (Sigh... although we use them for storing things, I'm not kidding... these things are the size of beach balls). You can add them to fruit smoothies or sprinkle (yuck) over your food.
Personally... I don't eat "snack food" for snacks. I eat left overs & appetizers/ cheese. Left overs are fantastic... they're already there! Appetizers are easy... because you can make a bunch and they're good for a couple days. Just snag from the fridge and heat up.