If it's raining and you are stuck at home with your toddler, and you fancy doing some painting, but it's too wet to even make the trip out to the garage to get a non-spill paint pot, simply cut a hole in the lid of a ricotta tub. Or any plastic tub you can find in the recycling. It worked pretty well - he only managed to dribble a small amount of water out of it, and he got on well dipping his brush in.
Incidentally in the photo he's using Ikea's coloured pencils that turn into watercolour paint when you brush over them with water. They are popular in our house at the moment - a good, fairly unmessy alternative to full-on painting.