I liked @luke396's lucid accomplishment about forcing a DC to age rapidly. Last night I got my 4 y.o. daughter's DC to grow until she was around 10-12 y.o. It was a pretty interesting experience.

I think it could be adapted into a more advanced task: Force an entire city to age rapidly for around 500 years or more. It should include not just DC's but buildings, trees, animals, plants, etc. The selected time should exceed the human's average life span and even any known long-lived time for a human on earth to see what your mind comes up with.