There is colouring, cutting and pasting, looking for simple patterns, pretending to be animals, and foolproof instructions to help you help your child play a couple of simple nursery rhymes (for very young children, they might rest their hand on top of yours while you play it, or you might help move their finger to press the right keys). Incidentally, we get to know our way around the piano!
You should be able to do this print out with your child even if you have no previous background in piano or music. It's pretty self-explanatory.
You will need a piano that has black and white keys. If you don't have one, K-Mart sells kids electronic keyboards for about $20 that are perfect for this kind of thing (although 'growly notes' will sound less growly).
So without further ado, here is a PDF file that you can download and print-out to entertain your older toddler / preschooler on a rainy day:
Toddler Piano Fun Free Print-Out
You can read it online but the quality is better if you download the file.