Interesting mathematic topics?

I'd have to say, I'm doing a maths degree and all of the things you've mentioned (with the exception of the golden ratio and Fibonacci numbers) have either not been covered or only briefly mentioned.

Perhaps you're not doing the right courses then? Some of those are covered in a third or fourth-year course in Cambridge:

Chaos Theory - Dynamical Systems (3rd year)
Zeta Function - Number Theory (3rd year)
Cryptography - Coding and Cryptography (3rd year)
Projective Geometry - Algebraic Geometry (3rd year, 4th year)

What is true, though, is that first and second year undergraduate mathematics tends to be rather less exciting.

Good point. I'm a 2nd year undergraduate in Warwick so that would probably explain why I haven't seen them yet.

To the OP: Maybe you could consider doing a three-dimensional surface plot of some function f(x,y) and calculating partial derivatives and marking them on the diagram? Then you'd have some maths that would be fairly understandable by A-level students as well as the nice pretty picture of the surface plot that would work well on a poster.

Perhaps you're not doing the right courses then? Some of those are covered in a third or fourth-year course in Cambridge:

Chaos Theory - Dynamical Systems (3rd year)
Zeta Function - Number Theory (3rd year)
Cryptography - Coding and Cryptography (3rd year)
Projective Geometry - Algebraic Geometry (3rd year, 4th year)

What is true, though, is that first and second year undergraduate mathematics tends to be rather less exciting.

Can you expand on this? Possibly give me some links explaining this stuff ;3

I'm not taking the course, but I suspect it's elementary algebraic number theory. We don't have an analytic number theory course, as far as I know. (Oxford does.)