Hi TooTicky,
I have lots of inquisitive 12 month olds still happily enjoying treasure baskets at my groups. We do normally have a brief hiatus when they first learn to walk, as that takes precedence over everything else, but after that they find new ways to explore the baskets. I think the guidelines Elinor Goldschmied gives are too narrow, personally - I started taking a treasure basket along to my toddler sessions, so the younger siblings would have something to play with, but the older children (up to 4 years old) all love it too. They use the objects in quite a different way as they grow older (more role play), but the open-endedness of it still fires their imagination.
At the age of 12 months, if you like the idea of scouring charity shops I would still go for making one, but there are various things you can do to extend the treasure basket activity for older ones. Get a few different sorts of receptacles to use with the basket, such as large tins (like a catering size coffee tin), straw baskets with handles, sturdy paper bags with handles, and maybe some shoe boxes, some with lids and some with holes cut in the lids for posting. This will challenge your daughter's fine motor skills as she posts, fills, stacks, removes and sorts, and allow her to indulge in every newly walking toddler's favourite activity - Walking Around While Holding Something
If like me you become totally obsessed with heuristic play you will find the next stage just creeps up on you gradually and your partner is regularly coming home to find your lovely new additions all over the house.
Me: Aren't you going to ask why there are 12 mug trees on the living room widnowsill?
Dp: No.
Me: Why?
Dp: Because I don't want to know.
Oh, boot sales are better than charity shops for the collections of stuff, and eBay is also good for collections (eg keys, ping pong balls)
Do you have a local scrap recycling centre? They can be brilliant.
Let me know how you get on and if you find any particularly rich veins of treasure