The treasure basket is not specifically a Montessori activity but it fits perfectly with the ethos of the Montessori education.
In fact, Treasure baskets belongs to the heuristic play.
I started to give a treasure basket to my children when they were able to to sit up by themselves (around 6 months old).
At first, it was a mix of objects made of different materials.
When they are older, I vary the kind of content to explore.
Here only metallic objects (very noisy basket!)
Only fabrics/soft toys/silk
Here Louis, 9 months at that time, exploring the basket
In the playgroup, this area is also very popular with the older children as they can imagine whatever they want with the content of the basket.
Adding some kitchen paper holder and rings, bracelets of all kind is also fun and note the natural items such as starfish or big shells.
Home made items are also a nice addition, just make sure, it’s safe to play with