My children have discovered a fun art activity that keeps them busy for hours.  They paint rocks.  So simple.  First, they go out in the yard and forage for today's art rocks.  Then I spread a large piece of packing paper on the table, and put a bunch of different colored tempura paints into blobs on a paper plate or in an empty egg carton.

They they go to town painting the rocks.  Flat ones are fun for little scenes and bigger ones are great for just colorful decoration.  After they dry, we place them back in the garden and they add a colorful splash.  Since tempura paints are washable, rain will eventually wash the colors off and you can start the process all over again.  (At least this is my guess).

Another project that I used to do as a kid was to "paint with water" outside.  My mom would give my sister & I a bucket of water and two regular paintbrushes and we would paint the garage doors.  (Back then they were made of wood and had flat paint that absorbed water so it was fun to "paint" so you could see it").  Since many garage doors are no longer made of wood, this would still work if you have a brick foundation, or perhaps a stone patio, sidewalk, or outdoor fireplace.