1. Packing cubes
2. Reusable grocery bag (that squishes into a small space).
Notice the little red blob in the top right corner of the suitcase? That is a reusable shopping bag. I ALWAYS travel with one. Sometime I use it as a purse cover of sorts. Stick my purse in that bag and then under the seat in front of me. I also use it to have extra room on the trip home or to combine a shopping bag and my purse into one "small" personal item when flying home.
Also, more and more places charge you for grocery bags, so this helps with that issue too when you are away from home.
This is really targeted to the ladies. I became a dry shampoo believer about 5 years ago. It is a travel staple for me. I can easily get away with second day hair which is a great time saver during travel. Plus if you are only gone one night and don't need to wash your hair you can lighten your load of hair products to travel with you.
4. Multi plug
Some hotels have one accessible plug near the bed and others hidden behind furniture or in the bathroom. I'm not a fan of charging my electronics on a bathroom counter. So I bring along a multi plug to turn one outlet into three. I used to bring along a power strip but this serves the same purpose and takes up less space. I found my current one from the Dollar Store but you can get them most anywhere.
I keep a set of disposable ear plugs in my carry on bag. I don't use them very often but on a late flight these are really awesome. Equally helpful if you happen to have a loud hotel. They hardly take up any space.
6. Oatmeal packet
Another item that takes almost no space but provides a great meal option. Heat water in the coffee pot and pour it into the travel cup usually provided in your room. Cover with the lid and let it cook for a bit. If you don't have a spoon the wooden stir stick provided for the coffee will do the trick. :) Quick, cheap and easy.
7. Tea bags
I have lived most of my life on well water. Chlorinated water tastes awful to me. So having tea bags with me is a great help in disguising the flavor of a bad hotel water. You don't want to get dehydrated on a trip. I've heard others mention this as a way to bring some sense of home with you. That can't hurt either.
8. Landry hamper
This is a fantastic find from the dollar store. It folds flat and takes up almost no space. It isn't the most durable thing ever but mine has lasted a year already. Totally worth the buck! I love having a place to keep dirty clothes away from clean ones. On car trips I have started just keeping the dirty clothes in the hamper and dumping it right in the washer when we get home. This is more essential for travel with the family than work, but sometimes those activities have some crossover.
9. Granola bar
Side story - when I was young some very close family friends got stuck in a snow storm in their car for several days. They lived off ketchup packets and miraculously survived. Since them my Mom has been a little crazy about emergency preparedness (and we keep ketchup packets in the vehicles still). Carrying a granola bar is a bit of that emergency preparedness. But you also might have a hangry mini emergency or get a bit nauseous and need something to calm your stomach. Granola bars last a pretty long time and take up hardly any space. So don't leave home without one. You can also sub a protein bar in here if that is more your thing.
10. Mint or peppermint candy
Peppermint is very soothing to a tummy and helps relieve nausea. Seriously it works for me. Especially on turbulent flights, mints are a must. I specifically prefer Mentos on flights, I swear it helps with ear popping since you kinda chew them.
11. Ready to go toiletries
This set of plastic bottles have been traveling with me since I was a senior in high school. These exact bottles. I try to fill them up when I remember after trips so that they are always ready to go when I leave. I don't have to worry about having to use hotel brand products and I don't have to purchase bunches of disposables.