Cheap Joe's used to sell an outdoor watercolor "Survival Kit" that was the perfect size for me when I took a group to Italy. It was packed with just the right stuff to paint anything you want and just the right size to go anywhere. I rode the vaporeti all over Venice with it and demonstrated for my class, too.
Now this great little bag comes empty and it's on clearance. You can pack it with your own personal collection of outdoor painting essentials.
If enough readers are interested, I can post a photo and list of my supplies that fit into it.