While most time capsules require a level of patience that's not entirely suited for a child's attention span, the Backyard Safari Time Capsule can be both buried and continually stocked with new items.
It's really a neat idea. You bury the $US20ish capsule, then an artificial rock hides a tube that reaches the surface. Whenever a child wishes to add more objects (baseball cards, LEGO minifigs, dead pets) they simply drop them down a hole.
Unfortunately, Joel from Boing Boing Gadgets observes that the container doesn't appear to be waterproof. In other words, the aliens who dig up your kid's capsule a thousand years from now will assume we lived in a time of floods, or at least that humans were very bad at keeping their dead hamsters dry. [bbGadgets]