I've been thinking about my sketchbook in terms of "how" instead of "what": I pick a medium or color palette I want to experiment with or improve at and dedicate the entire spread to that. It's helped me avoid succumbing to perfectionism-induced decision paralysis because I just think about cool stuff I want to try instead of getting stuck on making a pretty picture.
I've avoided dabbling in ink drawing without drafting with a pencil underneath because the thought of putting ink on the page without a plan fills me with irrational dread, but that changes now! (The avoidance is changing. No guarantees on the irrational dread.) This is also my first time doing plein air drawing, so there's excitement all around.
I always mix my own paint colors as a rule — I've owned only cyan, magenta, and yellow (plus black and white) for years — but I've been really into nontraditional primaries lately. It's an interesting challenge that produces interesting paintings, and the Zorn palette is a particularly... limiting limited palette.
The Zorn palette is named for Anders Zorn, a Swedish painter who used it extensively in his work. It consists of cadmium red light, yellow ochre, ivory black, and titanium white, a color selection that locks you out of blue and most shades of green. If you're using the Zorn palette for portraits, this isn't a major limitation, but landscapes are a different story (and guess what I've been using it for)...
I love tiny palettes. They're the type of thing that I see and swear I'm going to do [x] (paint) more because I have [y] (a tiny paint palette). I thought really hard about getting a fancy small wooden palette (as seen in this tiny palette mega review by Leslie Stroz), but decided against it in favor of engineering one out of a mint tin. I also love putting random shit in mint tins. I already keep my small cross stitching projects in one (you would be shocked at the amount of embroidery floss you can stuff into one Altoids tin).