I think the best brain exercise would be to learn a sufficiently different programming language. There is a perlis (Alan Perlis, not a typo for "perlish" :-) quote: "A language that doesn't affect the way you think about programming, is not worth knowing."
in reply to Recommendations for a self-taught Perl programmer
In that sense, perl, ruby, python, c, c++, java and c# are all too similar. (That isn't entirely fair, because they do have distinct idioms and features, but they are all close enough to each other that if you switch from one to another without being embedded in a culture to absorb the idioms, you'll carry over your existing habits too much).
Try learning scheme, haskell or ocaml and you'll get a very different perception of how to approach programming problems. In particular, follow a book or set of exercises in that language, since you're not just looking to repeat your current patterns of coding in a new syntax, you're looking to learn new programming idioms and ways of approaching and thinking about problems.
I don't know of suitable ocaml or haskell resources, but there are quite a few good scheme related books, notably the challenging Structure and Interpretation of Computer Programs - which also has downloadable lectures, How to Design Programs - which is more focussed on design/software engineering than computer science.
I worked through (most of) SICP, doing the exercises and asking for help on a yahoo study group when I needed it. Some of it was mind blowing to me at the time and I think it has helped me look at certain language features (list manipulations, closures, functional programming) in a new light. I don't mean those tools are always the right ones, but now I have those tools as well as my other ones. And the nice thing is that perl is a sufficiently advanced language, so you can apply them all.
In fact, coming back to perl from scheme, I can see that a lot of "advanced" lisp/scheme ideas were embedded in perl from the start. Clever Larry.