The best one I've ever found is a BIG clean empty box, with a few smaller and tiny boxes to go with it.
It is good for rainy day indoor play, for on the porch outdoor play, and for partner-play with little cousins or neighbor children.
The boxes are fantsy-land basics with the addition of 'small' plastic animals such as farm animals, or tea party stuffed animals, and doctor/hospital events. It can be a safety fort or a cottage - or a magnificent castle! The imagination of a child can activate the boxes into a whole new and complete world for those few moments of play - and learning practice.
The senario can be a rocket ship to space, a plane or boat ride to Grandma's, or a race in the park. Then after a span, it may also become a safe haven for a nap. Mommies will like this feature too!
Give the child a few boxes, and they can imagine the WORLD!