What are some learning ideas for a 13 month old who loves boxes, shape sorters, and such?

My little boy loves his shape sorter and enjoys putting objects into boxes and dumping them out. He is enjoying learning how things to fit in different places, so I am trying to come up with ways to encourage his exploration with common objects around the house. Today I gave him 4 plastic cups and he spent a great deal of time stacking (nesting) them together and taking them apart. Any unusual ideas for similar types of activities? (I also posted this in the parenting section.)