I recently stumbled on a source of art ideas in an unlikely place - the Staedtler (German pen-making company) website. It contains a number of tutorials and art/craft ideas but I haven't yet trolled it thoroughly.
https://www.staedtler.com/ca/en/discover/
Staedtler also has a mandala creator - create and print your own mandalas with lots of design options. This is a time sink if you are having kids create their own, though. I have used colouring mandalas with older students as an activity for their hands while we have a discussion (this works really well for shy students - no need for eye contact) but have yet to use them in an art lesson.