There are four different setups I use on painting trips, each one progressively lighter and simpler. Which one of these setups I choose depends on our destination.
The first setup consists of a full-sized French easel, a large palette that folds up onto itself like a suitcase, and a backpack that holds about twelve oil colors, my brushes, palette knives, solvent jar, paper towels, and canvases. I use this setup when we're painting within the US, have a rental car, and aren't going to be hiking long distances to find our subject matter. This is perfect for plein air shows like the Laguna Plein Air Painters of America and the Plein Air Painters of America show on Catalina Island. In both instances, I simply pack everything up into a large cardboard box and ship it UPS to the hotel I'll be staying at, avoiding all the hassles of checking it through on the airplane. This first setup is also ideal for local landscape painting or driving trips. It's nice having all my brushes, colors, and a large palette when doing larger on-the-spot works.
My second setup is for places that you can't ship your gear to ahead of time, and where you might be doing quite a bit of walking to find your painting spot -- National Parks or Europe, for example. For these trips, I usually substitute a large pochade (self-contained) box that holds all the paints, brushes, canvas panels, and other equipment compactly into one, easily carried unit. The box itself attaches to a tripod so I can stand and paint or simply sit, and it rests on my lap or on a table. This is perfect in Europe where you can sit at a café and paint while sipping tea! I don't usually do anything larger than 12" by 16" with this setup, and most of the paintings tend to be around 9" by 12". Everything fits nicely into a backpack with just enough room for a camera and jacket.
Now, for places that are a bit off the beaten path, like trips we've taken to China and Nepal, you really need to travel light. So, I use a third setup which consists of a small box, three or four brushes, and only four tubes of water-based oil paint (red, yellow, blue and white). We use water-based paint since it is nearly impossible to track down acceptable paint thinner in such places; and would have the extra weight of carrying a jug of thinner around all the time. And of course, paint thinner isn't allowed on planes! Water can be found everywhere, and it makes cleanup especially easy. To replace bulky canvas panels, we paint on 6" x 8" museum boards that we seal with gesso. Make sure they're not paper mat boards, the latter being acidic. These are so thin and light that you can easily take as many as you want; they come in many different colors, which can also be fun to experiment with. With a razor and some tape, you can easily construct a cardboard slot-box to hold the wet sketches, then simply stack the one that have dried. I've also occasionally used gouache and acrylics with this setup -- then you don't even have to worry about them drying. You might think 6" by 8" is very small, but you can easily get all the info you need to do a larger painting in the studio. If it's the choice between nothing and 6" by 8", which would you choose?