We buy big rolls of freezer paper at the grocery store and use this for most of our art projects. The shiny side is great for fingerpainting (or anything else except watercolors), and the matte side is great for watercolors. It's much less expensive than craft paper rolls.
Tape a large piece of paper to your work surface. Let your child gather a few small cars, or anything with wheels; ones with some kind of tread make the best tracks, but the smooth ones are fun too.
We use a muffin tin with applesauce cups stuck inside for our paint tray. Grab the paint brushes and "plop" some paint on the paper. Here M is making "puddles":
Take the cars and run them through the paint.
Put more paint down, grab another car, run it through. Your little one will catch the hang of this very quickly, believe me.
In the end you have a pretty painting like this:
Then, more fun! Stick your child at the kitchen sink full of soapy water and let him wash all the cars, paintbrushes, etc. while you sit back and relax. Yes, it's messy. Yes, you'll probably end up really washing the cars and paintbrushes. Yes, water might get everywhere. Put a towel on the chair, on the floor, and anywhere else you want and let them explore and have fun. The plus side is a little mess = a lot of time entertaining themselves while you relax, fold the laundry, whatever. :)
Enjoy your messy art time! And have a beautiful day! :)