While you're there, look at the weirdest thing in the produce department. Bright orange cactus? BUY one. Go home and get online and try to figure out what to do with it. Or just slice it open to see what is inside.
Or buy a coconut—shake it to see if it has liquid inside. Let the kid pound on it with a hammer until it cracks open. While they're doing that, do a quick google on coconuts so you have some background knowledge. Don't "teach" them—but if something seems cool, just say it as an interesting, cool thing to know, "Wow, coconuts are SEEDS! And, oh my gosh, they sometimes float in the ocean for years before washing up on some island and sprouting into a coconut tree."
SandraDodd.com/nest or Coconut
photo by Sandra Dodd