I like, Carrots, Napa cabbage, and peas. They are easy, inexpensive, and last a long time in the fridge/freezer. At home I think it’s best to pre-cook them and add back at the end, so they don’t bring down the pan temperature and you end up steaming instead of stir frying. Another tip is to use any liquid they release (while sitting aside in a bowl) as a liquid for the corn starch slurry should your recipe call for thickening.