I've only done a few, but ..
Topology: Probably the most interesting second semester subject, George tought it this year and apparently it's a fair bit different from last year. I'm not sure if you're more into pure or applied stuff, but it's mostly pure, but with a stack of interesting consequences (always a cyclone somewhere on the planet, always two antipodal points on the Earth's surface that are the same temp, and bunch of other 'maths' results that are interesting just because (imho
)).
Algebra: Think group theory from linear alg. It's that, then some more of the same (rings - extra binary op and another axiom) and then some more (fields - still two binary ops but a couple more axioms), and some interesting stuff in between (counting, impossible geometric constructions). I was actually looking forward to this one, but I got bored? Maybe it was just the lecturer (Andrew), maybe I suck at it, dunno, I just couldn't get into it. If you do this, I think the textbook was heaps better than his notes.
Topics in applicable: Ugh, this was a good subject. I liked it, but I just sucked at it. Do it if you're into the applied stuff though. Coding theory, bifurcation theory and game theory.
Combinatorics: I haven't done it, but it's due to run next year. Jim normally does it, and it runs second semester, but I think Jim is on leave second semester. Either it'll get moved to first sem, or someone else will take it I guess. I've only heard good things about this one though.
Logic and set theory: Due to run second sem next year and George is taking it. I'm keen to do this one out of interest, and hopefully I can next year, but I've heard it's pretty painful. Oh well.
I think that is everything that is running? I think linear ops is only every second year, but if not, that course was hardcore.
Oh, and I have the algebra and topology notes (from davids and georges course) if you want to suss it out, just post or pm your email or something and I can send them.