Painting plein air is so wonderful in the beautiful spring and fall weather and such a delightful thing to do in a public garden or just in your own back yard. But wherever you choose to go, hauling all your paints and supplies can be difficult and make the day just one big chore.
Here's what I recommend for painting plein air in acrylics.
Supplies you will need include: your paint
your brushes
a water supply and bucket to put it in
canvas(s)
a portable easel
and optional paper towels, and garbage sack, apron, and stool.
Here's a picture of the "pile" that you'll be toting.
Allow me to simplify this "pile" in my own Jane's Perspective way. First of all you don't need ALL of your paints. Concentrate on the color wheel as you pick a basic red, yellow and blue, then add a green, orange, purple, black and white. You can mix anything else you need from these colors and a regular size tube of each color weighs in at merely ounces so you're only up to carrying a pound at the most so far. I once painted on a beautiful spring day when the green grass was growing, the green buds were shooting up all around me, and the trees were covered in green leaves - so what color did I forget to pack? Green! So I mixed blue and yellow all day and got some fabulous greens. So don't sweat the hundreds of tubes of paints you think you might need. You can get by on way less and it can be a wonderful challenge to your color mixing abilities.Likewise for your brushes, pick out all the brushes you want to take and then cut your number of choices in half. Make sure to add a fan brush and a small house paint size brush (if you do a wash of sky like I do to begin with).
The easel should be metal or wood and should fold up easily and extend easily. To pick a portable easel, the best rule of thumb is to get the cheapest foldable one. It's hilarious to me how difficult so many of those high priced, so called "easy up" portable easels are to actually set up on location. Plus many of them are super heavy. So get a cheaper easel until you see how much more you actually need. Jane's Perspective is that if it ain't "broke", don't buy it.
Your heaviest piece of equipment will be the water jug. By the end of the day, this will also be your lightest piece of equipement and possibly discardable if there is a proper receptical nearby. I don't recommend using lake or ditch water - Blech, I can't imagine what that might do to your brushes or paints. To "kill two birds with one stone" I like to take either a 1/2 gallon or less size container of drinking water so that I also have something to quench my thirst on a hot day.
Your most awkward piece of equipment is your canvas(s). I like to go all out with a 16 X 20 or two but you can easily do smaller paintings with flat 8 X 10 or 9 X 11 canvases. Smaller canvases are packable while larger ones need to be carried seperately. I have a friend who makes canvas carriers for her oil paintings and they come in handy for oil paintings done in classes but for plein air, it's just another thing to tote plus this blog is about acrylic painting so our canvases can be stacked easier. They'll be dry before we even pack up to leave.
Paper towels, an apron, a snack or lunch, plus a garbage sack finish our the supplies but they can all be carried easily as you can see in this "gee I need to lose weight" picture.
Here's how I do it. In the garden section of my local super store, I found a folding stool that had an attached pouch. When you unfold the stool, the zippable pounch is as big as the whole underside. On the outside of the pouch were pockets for garden tools. Inside the pouch I was able to fit my paints which were stored in the waterbucket (an ice cream size plastic container) along with my water and apron, a small roll (not a new roll) of paper towels and a snack or lunch in a baggie that became my garbage sack. Jane's Perspective says to look for "double duties" for each item. Outside I slide all of my brushes into the pockets. Once folded back up, the stool legs became the handles. Canvases and an easel fit nicely under my arms and I'm good to go. That makes only three basic things I have to pick up to carry. If you can't find a stool like I use, a folding stool without the pouch can be slung over your shoulders OR you may not even desire to sit down while you paint. Another option to sitting is to carry a small blanket instead of a stool, spread it out on the ground, and use a table easel. With lunch, you'll feel like you are on a picnic. 
So there I am all set up and ready to paint - you can see the gardening stool I use really well in this picture. Oh, and check out the crazy sunbonnet. 