I remember when we were little, my sister and I used to spend FOREVER painting the bricks that surrounded the neighborhood pool with water. Of course by the time we made it around the whole pool, the water had dried so we had to start over. We had a goal and were determined to accomplish it! My mom was free to talk with the other moms and we were completely content. The paintbrushes were a permanent addition to our swimming bag for quite awhile.
So when J got old enough, I capitalized on this activity. I used to put him on our back deck with a bowl of water and a paintbrush while I cooked dinner on the other side of the glass doors. He was content (literally 3 feet away) painting the boards in the deck and I was free to cook without my pretoddler under my feet.
At two years old, J is still entertained with just a paintbrush and water. Sometimes he paints whatever he wants and sometimes I give him a goal to accomplish (paint all the steps, paint around the car).
This link and this link shows how to capitalize on a paintbrush and water indoors.
Age attempted: originally around 15 months, now 25 months
Great idea!!!