I agree with Eden that a wheel upgrade is a good place to start. You might look into getting a set of handbuilt wheels, which depending on the build can be cheaper than factory wheelsets but still provide a marked improvement in ride quality, weight, etc. An upgraded rear derailleur will also likely improve your shifting.

That said, I wouldn't necessarily encourage you to upgrade any of your other components just to upgrade. Instead, I would suggest either saving your pennies for a new bike down the road or waiting until something actually wears out--which it will with enough riding. This is coming from someone who upgraded things on her own bike. I don't regret doing it, but over time, I've gotten a better sense of just how expensive it is to maintain my bike. It adds up.

Good luck with your century!