Wills - one thing that my 2 adored as children (copied from the Montessori toddlers group we went to) was a treasure box.
We got a big wooden box and filled it with interesting things (nothing plastic) IIRC things like a tin whistle, small purse, pieces of different textured fabrics e.g. velvet, satin, organza, acorns, pine cones, old beaded necklaces, scarves, hats, shawls, miniture brass ornaments - especially kettles and teapots etc, china ballet shoes, china animals, polished stones, rings, miniture cymbals, leaves, little wooden ornaments, feathers - you get the idea!
This used to keep them occupied for hours and costs virtually nothing!
In terms of things to buy, I think shape sorters are excellent for this age group!