I add chia seeds to lots of things -- oatmeal, yogurt, salads, even desserts. They add a nice little crunch.
I've also started adding hemp hearts to a lot of baked goods, like banana bread and oatmeal cookies. They have a mild, nutty flavor that blends well with other things and are loaded with protein.
Greek yogurt of course.
Black beans (also a great salad addition, good on their own or with rice as a side).
Avocadoes
Chickpeas
Lentils
Cheese