Bring a measuring tape with you when you go shopping for luggage and count the wheels and any protrusions when getting the dimensions. Don't trust what the tags say! My (very old) inexpensive carry on is branded by an airline that I use frequently, and it has fit overhead and even under seat in planes small and large. It measures 20x6.5X14 and is a 2-wheeler. I have used it for everything from a 3 day weekend get-away to 28 days abroad.
I have a matching Lonchamps that I use as my second bag and purse. Usually I have a smaller dressy purse that I toss into the Le Pliage, along with my electronics, travel documents, spare glasses, etc.
Travel cubes are great. You can pull them out of the suitcase and just toss them in the hotel dresser.
Flip flops are flat, light, can get you to the pool or act as slippers in your hotel room.
Pack a tiny first aid kit in a sandwich size freezer bag and leave it in one of your inside suitcase pockets. (Bandaids, ointment, tiny nail clippers, q-tips, a few cotton pads, etc.)
Vivienne Files is great for travel capsule ideas.
