To that end, I, too, have a small kit I carry every day. It does fit in my purse/bag but sometimes it's still more than I want to carry.
I've recently purchased several of the Field Notes Workshop Companion limited edition notebooks because the paper is good for sketching. They are 6 for $20. It even can stand up to a light watercolor wash. They have a dot grid, which isn't ideal but is acceptable for these quick, 5 minute sketches. However, it is a limited edition and won't be available for much longer.
Then I read about this Etsy shop. YellowPaperHouse offers several sizes of small notebooks. One option is the Field Notes sized books. The paper choices include Canson Mixed Media and Watercolor papers. These are decent papers, especially for a tiny book meant for fast sketches. I made some sketches in both these books this weekend and I am very happy with them. I suppose you could make these yourself but at $6 each, they are fairly affordable.
These tiny sketchbooks make it even easier to sketch every day. One of these and a pen makes a good carry-every-day kit. Add a bit of paper with a few watercolor dots and a waterbrush and that's the enough to add some color to the drawings.