Very simple, but those stacking cups from ELC. They are about a fiver.
They are good for sorting into size so slotting inside one another, building into a tower by turning them upside down, and learning colours. You can also use them in the bath or in the sand as they've got holes in the bottom for things to pour out of and also patterns on the bottom so you can make impressions in the sand.
Personally, I think kids develop their imagination far better with simple things like a huge cardboard box and some kitchen roll tubes!
Much prefer that to any spangly, flashing light type toys. In fact, we don't actually have that many of those kind of thing.