They are TDD, OO-design, increase Rails mastery, refresh myself on algorithm basics (sorts, processing data efficiently, etc), and learn the basics of a JS MVC framework (leaning toward Angular). I'm not starting from zero in any of them.
Also, I did a poor job of providing context/perspective, so I'll quickly add some: My dev skills are at a point where I could get a junior Rails gig somewhere. I want to get them to the point where I can get a junior/mid Rails gig (almost) anywhere.
The 90 days was just an arbitrary time constraint that I felt was short enough to give me some urgency, but long enough to make significant progress.
Totally agreed on the "having a project" points you make, and on learning by necessity. Both are a part of my plan. I love learning about programming in general, but yesterday I found myself reading a Ruby book that was talking about making external DSLs and I had to stop myself: "This is really cool, but WTF does this matter to me right now?? Go read about TDD instead."
EDIT/ADD: Thanks for the reminder about spaced repetition btw.