Here are a few ideas I've turned up over the past few months. If you have any good ideas....please share!
"Color Boxes" Big Kids Can make for Babies
"Touch-and-Feel Picture" for Younger Siblings
Older kids can design a "touch-and-feel" picture for their younger brothers and sisters, or even make a whole book. Nora worked on this picture for her little brother when he was about 16 months old. He loved it, although it's the kind of thing you'd want to supervise a younger child with (I don't think 3-year-olds really understand, you know, safety codes when making a toy for a younger sibling, so we glued things like craft pasta onto Soren's picture -- obviously, this item would be recalled from Toys R Us!...). But, Nora enjoyed presenting Soren with her picture, and his little fingers were busy exploring all the fun textures.
Play Tips for Different-Aged Siblings
Games "Big Kids" Can Play with Their Younger Siblings