Busy boards are all the rage for younger children. Using different textures and items to give your children an opportunity to practice opening and closing things, making music, and exploring.
But they cost an absolute fortune and, quite frankly, you can make one yourself with things you have lying around the house! Pots, pans, string, pieces of odd-shaped wood. Attach them all to a pallet, and make a sound station for your children. They can even help design it!