One of my upmost favourite things to paint on is wood. I love the effect it has on the brushes and I love how when the paint dries you can see the wood grain underneath. It also gives the watercolours a beautiful waxy matte feel that cannot be beat, in my opinion anyway!
I don't only paint in watercolour but it sure is my main tool these days. I just love the fluidity and grain.
It's a tricky substance and especially on wood you need to prepare it sometimes with a quick sand, acrylic gesso, another sand and off you go! (Acrylic Gesso on anything makes it able to watercolour BTW!)
Here are a few examples:
Wood Slate + Acrylic Gesso + Acrylic Sealent
Wood Slice + Sanding + Watercolour + Matte Glaze