You know, one of the things that eats up my trip budget (other than gas) is snacks. When you travel, you have to keep hydrated and get snacky in between departure and destination. Convenience stores are the most convenient place to stop -- you have to gas up and go to the bathroom, so it makes it one stop instead of two. Of course, that means when you are thirsty, you pay a premium for pop and chips.
One thing that I do is I grab some coupons for drinks or snacks at other stores (or even other convenience stores) and keep those coupons in the car. That way, when I hear "Mom, I'm thirsty!" I can look through and see what I have. Sometimes a convenience store is actually the cheapest place to use a pop coupon since they occasionally run a 2 for $2.22 deal on certain brands. Combining it with a coupon just makes it all the sweeter.
You may not immediately think about using coupons at a convenience store, but they do take them. The same is true in reverse - sometimes you'll find a great coupon for .35 or .50 off a product, but using the coupon at a grocery store that doubles can get you the best deal. When Shoppers was superdoubling a year ago, I got several free Dr. Peppers that way.