Blackboard paint tree slices for natural mark making on the go. Great outdoor mark making idea. Kiddies could do this before working in their sketchbook. A basket of these for children to take out and explore.
Coral formations. German biologist and philosopher, Ernst Haeckel, mid 19th c. This is a black and white plate illustrating the beautifully detailed forms of a number of corals, from his Kunstform der Natur.
Cut metal house numbers. This one's from Etsy and looks close to a lot of the high-design jobbies flying around. It'd be easy to reproduce with the help of your local iron working shop. What? Doesn't everyone have one of those down the street?