Quart size double zip baggie
Green crayola finger paint (the key here is water based)
Clear packing tape
I put the paint in the baggies, got ALL the air out and zipped it shut. Taped it on the top and bottom to the counter (horizontal) and then used smaller strips to cover it (vertical). There is NO WAY that paint will get out of there and it entertained him for a full 15 minutes before moving on to another task.
I left it on the counter (been there 3 days) and he has played with it a few minutes here and there each day.
Definitely a fun, easy activity!