The Electrochemistry Lab is ideal for your older students to do while younger ones watch. Make sure there is still an adult present to supervise though!
When I was a child I really enjoyed rocks. Whenever we would go to a mineral store or a national park I would buy a few shiny rocks for my collection. One of the things I always wanted was a rock tumbler to make my own rocks. When our second oldest son showed an interest in rocks we decided to get him a tumbler so he could make his own.