So, let's see, Mexico. Swimsuit, shorts, shirts, flip-flops. A fleece in case of rain and for San Francisco, a couple-a-three books, my iPod and camera. Ready... set... GO!
In one hour I pulled it all together and am ready to go. Boarding passes printed, passport in hand.
I use a North Face Terra 40 summit pack (the same pack I use climbing). It weighs about 2-3 pounds, and is really versatile. I also like the Eagle Creek Pack-It Folder 15, and a Half Tube Cube. The folder holds shirts and shorts, the tube holds underwear and socks. I wear Patagonia shirts because they are loose, comfortable, and you can wash them in a sink and hang them out a window to dry, and they still look great.
Carry on luggage requirements vary from airline to airline, but in general US airlines allow you about 50 pounds, and about 24" x 15" x 10" size.
European airlines cap the size at no larger than 55 cm x 40 cm x 20 cm and may not weigh more than 8 kg. That is 22" x 14" x 9" (total is 45") and 17.5 lbs in Americanese. (10KG = 22lbs)
So my pack fits these smaller size requirements.