I would like to add this related question to the discussion: what resources/courses/learning techniques do you and others like you who have had an undergraduate

I would like to add this related question to the discussion: what resources/courses/learning techniques do you and others like you who have had an undergraduate Computer Science degree for a few years and are interested in academia but not sure what steps to take next use to continue your study? At some point I would also like to return for a PhD or Master's degree and I would like to keep myself mentally trained, but there are so few resources for higher-level study. All of the online resources I've tried are almost exclusively introductory and don't provide the challenge I crave. Does anyone taken higher-level courses at a local university in their free time just for the experience and to be surrounded by like-minded individuals while having a professional career?