PROMYS is a great way to be exposed to math concepts that aren't ordinarily taught in school (e.g. quadratic reciprocity, Pick's theorem, Minkowski's theorem, Hensel's lemma) and to the art of proof-writing. IMO the seminar in abstract algebra is a wonderful complement to a lot of the number theory material.
"The thing that doesn't fit is the most interesting."
-Richard Feynman