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.
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"The thing that doesn't fit is the most interesting."
-Richard Feynman