If you are really stuck with eating fast foods look up the nutrition guides online. Most places have them so you can make an informed choice and look at the carbs. Skip the fries but have the cheeseburger just order it without the bun. I have done this and they never say a thing. For crunch replace the chips with pork rinds. You might need to try a few brands and a few flavors until you find one that suits your taste. I have these with a little salsa when I have needed to munch. Almonds are also a good Protein rich snack. You can nuke pepperoni pieces in the microwave to make chips. You can even nuke an american cheese slice cut into squares to make a cheese cracker. Go for the SF frozen pop, Jello, pudding cups, frozen fudge pops or something like an Atkins bar for your sweet tooth.
Eating low carb takes some planning but with a little time you can figure it out.