A small tripod magnifying glass. You can get some with the same magnifying power as commercial microscopes and sufficient space underneath to direct a flashlight at a specimen.
For younger children, prisms, rocks for a rock collection, magnets, Brain Quest cards, plastic test tubes/beakers/graduated cylinders (can wrap in a paper towel tube to disguise), flashlight, polyhedral dice for math games, Schleich figurines, a mini weigh scale, a desktop anatomy model (some come in 4-6" formats with removable pieces for ~$20), a stylus for drawing on an iPad, coins or stamps for collecting.
For all ages-- a membership card to a museum, tickets to a special concert or event, small books, chemicals and test strips for experiments, high quality binoculars, a travel Scrabble game, gift card for iPad apps, tools for people who like building electronics/robots, an iPhone or iPod, high quality headphones (Skull Candy sells a set I like for ~$25), art supplies (charcoal, turbillons, water colour pencil crayons, pastels, fountain pen and ink).
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