I have a couple of ideas for reusing old piano keys. 1) Try painting dots to make them look like dominos. 2) Drill a hole in one end, paint it, or decoupage pretty flowers or other favorite things, and then string a ribbon or a cord through the hole and wear it as a necklace. I've seen some really pretty necklaces made from old piano keys. Good luck and happy recycling.
How about finding a piano restoration specialist. He or she may need this old ivory. Also depending on how thick the ivory is. It might be good for carving guitar parts. You could try your hand at scrimshaw too.