The treasure basket is a very specific activity with the goal of exposing the child to as many different sensory experiences as possible.
99% of all plastic objects feel, smell and taste exactly the same, and sound the same when dropped. Yet babies and children are mostly surrounded by plastic items and given plastic toys to amuse them. The treasure basket is an antidote to the overwhelming tide of plastic crap, and should IMO not contain plastic if it can be avoided.
Natural objects: Pine cone, large pebble, loofah, large shell, pumice stone, large feather, large cork, natural sponge, citrus fruit.
Wooden objects: egg cup, clothes peg, curtain ring, nail brush, small bowl, coaster, wooden spoon, napkin ring, small ornament, small lidded box.
Metal objects: spoon, bunch of keys, egg whisk, small sieve, length of chain, jar lid, small bowl, curtain ring, bracelet, bicycle bell, egg cup, bulldog clip, lemon squeezer, garlic press, small mirror in frame, metal keyring, tea strainer.
Aromatic objects: Citrus fruit, leather purse, lavender bag, rubber door stop, leather glasses case.
Noisy objects: Bell, whistle, piece of cellophane, chime ball, harmonica, castanets, small maraca, other shakers made from wood or metal, bean bag, rattle, bunch of keys, tin or box filled with dried beans and glued firmly shut.
Fabric pieces: scrap of velvet, fur, silk, corduroy etc.
Other interesting items: paint brush, toothbrush, ribbon, small thick glass jar (eg individual sized jam jar), marble egg, empty salt pot, string of ?pearls?, raffia mats, small baskets, shaving brush, wicker ball.
The basket should ideally be shallow, but large and sturdy enough for the baby to prop himself/herself up against, so that their hands are free to explore. But anything you have will do for now.
These items have been selected to be safe for babies under 1 year to explore with supervision. But every child is different - use your knowledge of your child to judge what is safe, and never leave them unsupervised with the basket.
I have cut and pasted huge swathes as above from posts over the years by Franny, our very own MN Queen of the Treasure Basket.
HTH