I typically do wraps of some sort for lunches.
You could also eat all of that on crackers, or some people carry mini-bagels.
Otherwise, I usually go with heavy snacks as everyone's mentioned.
On shorter days where you can do a longer lunch, it is nice to pull out the stove and cook a freeze-dried meal or something that takes a bit more time to throw together.
I am with @Wanderer . Flour tortillas with peanut butter and jelly for me. It provides the satisfaction of a sandwich with a nice carb load to get you through to dinner.
Lunch is always a time for a 'treat' as well (mine is red vines).
*Tip* I load up on free jelly/jams from hotel breakfast bars. My peanut butter source is usually Justin's although you can often find some of these in hotels as well.