One important category of item that I always have in my bag during travels is a small collection of plastic bags. They come in handy for two main purposes: to keep stuff dry and to keep your things organized.
When you’re out and about on vacation, chances are it’s going to rain at some point. Whenever this happens, it’s nice to have some sturdy Ziploc bags available to protect your electronics. Many phones released within the last couple of years are water resistant, but even with those I’d rather not expose them to water unnecessarily if I can avoid it. The phone that I currently have (the iPhone 6S) is not officially water resistant, so having a small plastic bag for inclement weather is a must for me. I also carry around a larger gallon sized storage bag for my Sony RX1 camera which is too big to fit inside a standard sandwich bag, but fortunately I have not yet been stuck out in a downpour carrying that particular camera.
I also keep any liquid toiletries I have in plastic bags, and not necessarily due to airline restrictions. I’ve had sunscreen bottles, bug sprays, and other various fluids leak inside my backpack, and it’s no fun to deal with, especially when you’re on the road. If you own high quality travel bottles, those rarely leak, but stuff like sunscreen is just more convenient to leave in its original container.
In addition to some different sized Ziplocs, I also have a few regular plastic grocery bags in my usual travel luggage. These are useful for separating dirty clothes in the off chance that I do not have the opportunity to do my usual nightly wash of worn clothing. You don’t want to mix your dirty clothes with your clean ones if you can avoid it, and if you travel with only a small backpack like I do, there are only so many compartments within the bag where you can effectively keep the two separate.

If you find yourself needing a larger sized bag and forget to bring one, most hotels have a convenient laundry bag hanging in the closet that is meant to be utilized for in-house laundry service. I’ve found those a perfect size for carrying around dirty shirts and undies.
This is another item category where I am happy to pay a bit extra and spring for the name brand bags like Ziploc or Hefty. They’re sturdier and seal better than the cheaper generic plastic bags, and I don’t want to have to be dealing with a wet phone while on vacation because I wanted to save a little money on the cheapo brand.
Plastic bags: useful, versatile, light, and inexpensive. Perfect for the light traveler!