There are lots of different I Spy ideas. Some use novelty prints that a child can easily Spy. Some have 2 of each novelty prints that can be matched to Spy. I have seen one on The Quilt Show site (I think) that was a game. The game had bean bags that each child tried to throw and land on different I Spy blocks. Some people use different shapes and coloured fabrics in an I Spy. Some are gender specific like Girls: dolls, ribbons, flip flops, crayons, etc. Boys: baseball, frogs, dinosaurs, cars, etc. I did see a site once where a list was provided with the novelty prints for the child to look for.
Let your imagination be your guide!