I use my iPhone to make those notes. My purse is too heavy to add something else to it, plus I'd probably forget the Doodle somewhere. Any Smartphone would work for the notes.
When I want a visual of something, I draw it in Paint on my laptop, save as a .jpg and sync it with my phone, but it is whatever works for you. I was considering buying a Wacom tablet to load my finished tops on to decide how to quilt them. Isn't it funny that we need more gadgets the more we quilt?