Q:

What are some interesting mathematics books/topics that are not usually covered in a standard undergraduate curriculum? 

Last term I started a weekly seminar like thing.  The basic idea was this -- At the beginning of the term each person will choose a topic that they don't know about and isn't usually covered in the standard undergraduate math curriculum.  They'll work on it throughout the term and present it at some point during the term.  Overall it was a success as most people involved got exposed to mathematics that they wouldn't have otherwise.  
Last term I suggested topics to each person one by one based on their preferences and experiences.  For example some of the talks we had were on: "How to have fair elections - Cryptography and Voting theory", "Quandles - The algebra of knots", "Hypercomplex Numbers", "Basic Elliptic Curves", "Ultrafilters and Non-standard analysis". 
However this term I am struggling to come up with topics for the 20 or so people that have signed up to speak.  So I'd like to ask if you have any suggestions for topics that are interesting and accessible to 2nd, 3rd year undergraduates but that are also not covered in a standard undergraduate curriculum. 
PS So far for this term I have "P-adic Analysis" (the book by Katok is great), "Bernoulli Numbers and their Application in Number Theory", "The mathematics of Bitcoins", and "Penrose Tilings".  I also understand that any book in the Student Mathematical Library will work.    

A:

You might try pilfering a few topics from summer schools (particularly ones your students don't have access to, for whatever reason).  For example, the LMS Summer School page has links to what was talked about in previous years, and for some you even get free lecture notes. 
Among the topics listed there, I would recommend (mostly out of my own interests):

Continued fractions and hyperbolic geometry, taking a brief look at the geometry of continued fractions via the Farey tessellation (see also Francis Bonahon's page for some very pretty pictures; he has published a textbook about this kind of thing). 
Quivers and Platonic solids, linking quivers to the Platonic solids via Gabriel's theorem. 

In particular, a set of notes for each of these topics can be found on the LMS pages linked above, which include references to other useful sources. 

A:

Going a bit broader, here are a few super cool fields of mathematics that have important connections to other fields but are often overlooked.  An introduction to them, and then proving a cool theorem might be a good assignment. 
Chang and Keisler's book on Model Theory, which is a branch of mathematical logic that has a lot of applications in philosophy and linguistics.  It also has an exciting intersection with combinatorics.  One of my favorite proofs ever uses the Model Theory to prove the Ax–Grothendieck Theorem. 
Reverse Mathematics (two essays: introduction and advanced) is a branch of mathematics that answers the question: why does it make more sense to say "Use the Bolzano-Weierstrass Theorem to show that if $f : [0, 1] \to \mathbb{R}$ is continuous, then f is uniformly continuous" then it does to say "use the four color theorem to prove that there are infinitely many primes"
Measure Theoretic Probability Theory is the proper way to do probability on infinite sets.  It will introduce you to the wonderful and weird world of $0-1$ Laws too. 
For more stand-alone topics... fractal dimension, Unique Games Conjecture, Minkowski's Theorem and it's many applications to Number Theory, a second shout out to using Model Theory to prove the Ax–Grothendieck Theorem, the connection between category theory and functional programming.