Yesterday I bought a set of watercolours, a box with 12 half pans that can hold 12 more. They are great if you want something more transparent rather than the opaque gouache. So now I have quite some options with which to fill my sketchbooks. I can choose one or the other or combine them. Or combine the watercolours with coloured pencils.
I also bought a watercolour sketchbook that I will use when my mixed media sketchbook is finished.
All I need now is lots of ideas what to paint.