Pretty much all I eat during my lunch break every day is sandwiches. I've kind of gone crazy with them. This is probably my favorite however, because it's delicious and very good for me. It's not as cheap a meal as mac & cheese, but still not bad for eating healthy, something I find hard to do cheaply.
2 slices of bread
2 slices of American cheese (optional)
alfalfa sprouts (optional)
about 1 tbsp of hummus (my favorite flavor is "garlic lovers")
spicy mustard (of course any type of mustard will do)
5ish slices of cucumber
I'm figuring this is pretty self explanatory. Cut up the cucumber, put it in the sandwich along with all the other ingredients. Delicious. Experiment with other healthy vegetables.