I use avocado in place of mayo on a lot of sandwiches.
I also use whole grain pita instead of bread a lot.
Other than that, just about anything goes. I think my favorite (or at least the sandwich/pita I make most often) is some variation of this:
Lean turkey or ham
Baby spinach
Cucumber
Sprouts
Sliced avocado
Sliced red (bell) peppers
Spicy or Dijon mustard
sometimes a slice of cheese
Sometimes I'll make either tuna or chicken salad out of canned tuna or chicken, a tiny bit of low fat mayo (just enough to hold things together) and then again, some variation of the above. Basically any veggie that gives me some crunch will make me happy~!
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