D.K.
Beans, chick peas, lentils, peas, edamame - we just pop these in a container just like the fruits and veggies for DS's lunches.
Nuts - almonds are a favorite but there are lots more.
Potatoes have only 4.3 g of protein for an entire medium potato - not awful but they have a high glycemic index and not much worthwhile for the calories.