How about some kind of old-fashioned toy -- maybe one of those wind-up frogs that hops, or a top, or something like that? Kids are fascinated by the concept of toys could have existed that weren't run on batteries, or powered by computers...

A harmonica or kazoo.

Old/vintage children's books are always fun, too.

Or some "vintage" clothes to use in "dress-up" games. My daughter always used to love to try on my high heeled shoes -- back when I wore 'em. But this goes along with Marsha's jewelry idea -- could be a long strand of fake pearls, a pair of long white gloves, big clip-on dangle earrings, a cool hat with a veil, feathers or birds or fruit on it, or a cloak/scarf/shawl.

Seed packets they can plant and nurture. Or a potato they can cut up and plant in water.

A kid's card game.

A collection of old postcards.

When I was a kid, I used to love those old Viewmasters, and the image disks that went into them. Loved that the photos looked "3D." Wonder if you can find any of those anywhere?

The ingredients to make homemade Playdough.

A collection of sea shells and a book to learn about them.

Small pair of binoculars and a bird-watching book.

An old cassette recorder and a bunch of old tapes from the 60s, 70s or 80s.