I used to play with beans, pasta, buttons, and beads, to arrange them into patterns. I'm not particularly artistic, but the patterns were fun and interesting. I still play with little colored candies that way when I eat them.
I went through the usual kid medical phase with extreme enthusiasm... I read books on medicine, nutrition, first aid... I remember being about nine, and reading the autobiography of somebody who had a spinal cord injury, and thinking she must be very ignorant not to understand that the spinal cord is not the same thing as the vertebral column. I also learned about sex and pregnancy from my mom's books. I think I must have been about the only kid who knew what Pitocin does, what a refractory period is, or what's dangerous about monoamniotic/monochorionic twins, but still didn't know about oral sex.
I made lists of the short segments on Sesame Street to try to determine which of them was played most often.
I memorized the entirety of every play I was ever in... not just my part but everybody else's, singing and speaking, and recapped it for anyone who could be made to sit down for it (not too many people, admittedly.)
I kept a drawer full of random objects, some of which would be considered trash, some of which were useful, some of which were just pretty. One day, my mom cleaned it out, and I was heartbroken.