I feel like I need to get the foundation that most people get in a BS before i can answer the question specifically. At this point I only know that I have a strong desire to know how computers work on a deep level.
Uff, there's so much to go through there. It's turtles all the way down.
Start with a course on computer architecture / digital design. Something that'll have you put together a simple 8 bit CPU. Move your way up with a course on operating systems - processes, swapping, cache invalidation, file systems, drivers.
Then, maybe something on compilers? Or how to write your own interpreter.
Finally, a good networking course is pretty important.
Very little of this is theoretical compsci, but it's something that will get covered in a degree program. That said, knowing how to recognize what kind of class of algorithm we're looking at, and data structures all around.
There isn't really a standard foundation that everyone with a BS in CS has. Curriculum are different and different colleges will emphasize different things, not to mention that many courses are elective.
It also depends upon what you consider to be a deep level?
Are you interested in electronics and signal processing, mathematics or the structure/design of programming languages? Are you interested in individual computers or distributed systems? Are you just interested in computers or are you interested in using computers to model other things, like economics, physics or virtual worlds?
At a minimum you could brush up on algorithm / data structure understanding and some of the math behind that. Problem is that as soon you gain understanding of something it will simply expose you to deeper subjects that you don't fully understand. You will never reach the bottom of all of these rabbit holes.
I have no illusions that I could possibly learn it all. I just want to learn enough so that I can go deeper on something more specific, such as virtual worlds, and no how to proceed.
Having a general CS education does not necessarily make you able to explore arbitrary fields. CS programs are ultimately designed by course administrators and professors and can be slow to change, so there's no guarantee that they will prepare you sufficiently to explore whatever avenues you wish to explore later.
Ultimately it depends on where you see your shortcomings that are preventing you from doing what you want. Do you wish you had better credentials so that you could get a job with better pay or more interesting work? Or do you find yourself frustrated because you are not able to sufficiently grok a certain subject area.
