Effective use of extra time during a PhD
Very interesting question, the answer of course depends on you and your preference. A few pieces from a Y3 postdoc...


Advice 1: Think of a skill that you feel as your biggest weakness. If possible, try working on improving it. This can be a management workshop, a presentation class, whatever. As a postdoc, having a well-rounded skillset is going to be useful.
Advice 2: Think of an area adjacent to yours which you considered to learn about, and then try to take an online course or something like that for it. Having broader overview helps a bunch.
Advice 3: Present your work to others. Write a bunch of profs of interest that you'd like to visit their lab and give a lecture about your PhD topic. But also, consider giving public-oriented "outreach" lectures, so that interested general public knows what's happening in your field. One feels this to be  important, given we're usully paid from their taxes.
Start writing that book that you always wanted ;)