Take a task of some inherent complexity but that can still be coded in 10-20 pages of code or less, create one solution, and then try out various different versions of it, or even complete rewrites, changing coding style, decomposition into modules, performance tradeoffs taken, data structures chosen and so forth, and then compare those with respect to length, code clarity, create benchmarks, etc.

Example projects: ray tracer, hash table (way more choices and trade-offs to be made than you would expect), toy grep, toy pseudo-pascal or lisp compiler, basic web crawler, ... It's hard, but you will learn a ton. Even putting substantial effort into improving a one page program, incubating and trying out ideas over a few days, can be beneficial if there is some "meat" in that program to chew on.

In general I believe software architecture is much more low level and less philosophical than a most aspiring architects believe. It's really about knowing the trade-offs of a decision before making it, and this rests on knowing a lot about data structures, algorithms, existing components (like databases, operating systems) etc. Those little programs in non-trivial problem areas often already show up a lot of those things.