A shake between meals, I use Trutein in water pretty regularly at work.
Either a home made or quality protein bar.
Leftovers! Cook a little extra dinner and Tupperware is your friend the next day, I do this fairly often when my fiancé can't finish her dinner haha
Canned turkey or chicken, I keep low sodium versions of both around.
Hardboiled eggs are simple and portable
Greek yogurt or cottage cheese, easy to find in single serving containers now as well.
Some notable vegetables for protein content: peas, beans (roasted chickpeas are a great snack you can flavor however you want), Brussels sprouts are a favorite of mine, equal carb/pro ratio and you're getting your greens + fiber.