English Paper Piecing is a great on the go project, but you need to carry it with you all the time. Sometimes you forget it (while you're grabbing the million other things you need) or you might be somewhere where you didn't realize you'd have some time to kill - or your fingers just need a break.
For me there are three essential quilting on the go tools that I always carry: a pencil, sketchbook and my camera (in my phone).
The camera is great to snap photos of random things that may inspire a quilt (or other project). These photos were all taken on my last vacation in a newspaper, at a gas station, tile in our hotel bathroom and a room key card! It's pretty easy to just snap a quick picture to help you remember.
Besides the camera, carrying a small sketch pad and pencil is essential. I worked on a design for a simple quilt back for my latest quilt while waiting for friends at a park play date.
I quickly designed a simple back and determined the size of the blocks. Next I figured out how to cut it quickly from the yardage so that when I got home later, I just cut the pieces quickly and got working to assemble the back in less than 30 min.
Another great use for a sketch pad is to practice drawing out some free motion designs. Drawing the designs really helps you know where you're going to go next. I practice drawing for quite a while before I attempt it on a quilt. It makes it much easier to quilt then since I've practiced the pattern so much on paper that I don't even really think about what I'm doing.
Happy on-the-go Quilting!