I am still in my undergrad program, but I like to do extra exercises when my homework is done just to really polish up my theory and stuff. Usually depends on what courses I'm in the process of taking, but some of my favorites to go back and look at are combinatorics, number theory, graph theory, abstract algebra and topology.
Unfortunately, my undergrad doesn't include/offer courses in complex analysis or topology, so I chose to audit the latter to get some formal instruction this past semester. Sadly I had to stop sitting in after the first month and a half, it was just too much with my other courses; on the plus side, I have the rest of summer to now go back and review everything I missed at my own leisure, so that's what I'll be up to until mid-August. Then it'll be on to graph theory from this book to prepare for a fall course I'm signed up for.