After purchasing a couple of new storage/organisation system draws for our upcoming year, I tore open the box, handed the kids some felt pens and let them at it. This was left out for a few days, the kids would come and go as they pleased. Added some doodles here and some scribbles there. It was way more fun having a huge textured canvas, than a simple boring sheet of white A4 paper.
For a bit more active fun, we grabbed some bubble wrap, the one with the really large bubbles. They enjoyed jumping up and down, feeling and hearing the bubbles pop and squish beneath their feet. Not to mention it allowed them to get rid of some of that energy from being cooped up indoors due to the rain.
Simple and nearly free to boot!