I have just one grandchild and when he was born could not wait to create his own toy box at our home for when he came to visit. He's four now and last time he came to stay I realised he had outgrown pretty much all of the contents of his box and it was time to hunt for exciting replacements for his baby toys.
As a mum of 7 I learned long ago that a good toy collection should include indoor and outdoor toys, creative, arts and craft and building toys, toys which stimulate the imagination, books and at least one cuddly toy. And the good news is, collating a collection doesn't need to cost a fortune
Firstly you need a "box". I have an old wicker chest but lidded plastic boxes or even a large sturdy bag (like the reusable ones you get at Ikea or supermarkets) will do.
Brio (or the cheaper versions of wooden trains and track), Lego and Duplo and Stickle Brix are great as you can pick them up cheaply in charity shops and car boot sales and will have a relatively long life in the toy box. The bag of Duplo I have was one thing which survived my toybox cull.
Books can be bought cheaply from places like The Works or charity shops and can be donated when your grandchild outgrows them. Cuddly toys too can be inexpensive and if you are worried about the hygiene of second hand cuddlies, choose ones which can be washed and in case of mites give the toy an overnight stay in the freezer.
You could stock up with some colouring books and crayons or pens but if you are concerned about paper waste or creativity extending to the walls/furniture/cat you could invest in a reusable sketch pad like the my first sketch pro neo colour LCD sketch pad perfect for children aged 3+.
