I saw this idea on FB this week and we tried it. In a one-quart mason jar, add dressing first. Then add anything you like - peppers, cucumbers, radishes, mushroms, cranberries, sunflower seeds, ch...
I set 5 jars on the counter and fill them all at the same time, thus 1 mess to clean up. The jars fit nicely in my refrigerator drawer and lunch is ready to take to work. They last all week, but I add dressing after I dump it out. If you wish, put a cupcake paper on top of the lettuce and set a hardboiled egg on it or drained kidney beans, etc etc.