Here is an easy way to make a light box for your child to experiment with:
Line a clear shoe box sized container with waxed paper and tuck a strand of Christmas lights inside. After you snap the top on, leave a small end of the strand and plug hanging out of the side of the box. Plug it in and have fun! For more details on how to make this diy light box, visit No Time for Flashcards .
Glass Beads are a fun item to add to a light table/box. This post at Teach Preschool also shows how to make a light box out of a strand of rope light and a scrapbook paper storage container.
You can use sand to create different designs and pieces of art on your light table/box like they did at Casa Maria's Creative Learning Zone
Discover with colored water as shown by Rhythm of the Home. A TON of awesome light table ideas at this site including colored shaving cream, mirrors, colored pasta, painting, and water beads.
Use bottle caps on your light table/box like they did at Teaching 2 and 3 Year Olds.