How about including the time of day in the game environment. Perhaps players could set their own clock based on timezone, and then the sun would come up and go down accordingly?
Also, it might be interesting to have some rainy days, or cloudy days where you can see shadows, or some other weather effects. I don't know if changing seasons would be too ambitious... how would winter work?
Just a couple of thoughts.
**EDIT - Oh, and how about being able to name the animals on your farm?