Just smile! :) As you pass people, look them straight in the eye and show those teeth! :) It might feel weird, especially if you live in or, like me, recently moved from New York, but if you think about it you know that you appreciate being smiled at. Who knows, maybe someone didn't see a smile all day except yours.
If you're bolder, you could even say hi, good morning, etc. or even give a random stranger a compliment.