I think one property people point to when they refer to subjects as "abstract" or "hard" is that they aren't very amenable to such micro-projects. Some examples of subjects like this include math, chemistry, and microbiology. There might be lots of applications you can pursue once you've developed a sophisticated understanding of the subject. If you're able to get an entry-level job in the field, you might be able to get lots of exercise on a small subset of the skills that go into the subject. If you're willing to spend a lot of money, you can sometimes buy the equipment to do at least some practice with these subjects at home, though you might also face regulatory and legal barriers. Perhaps you can find tractable, fun projects that are thematically related to the subject, but they're unlikely to precisely and comprehensively target the material on, say, the level of chapter sub-sub-headings.
Usually, curriculum designers and teachers tackle this challenge with homework problems, exams, labs, debates, and classroom discussion. But there are two problems. First, a self-studier might not have access to all those resources. Second, those tasks often lack the juicy, real-world applied interest that creates a genuine enthusiasm in the student.
If you've had some success with this challenge, or know of useful resources for exploring it further, I'd love to hear about it!
Note: All subjects and experiences are welcome. It's OK if answers stray slightly off-topic or lead off in different directions.