What to do in undergrad to make myself a strong PhD applicant (STEM)

I'm transferring to UCSD this Fall and I know I want to do a PhD after I graduate. I'm curious what I can do to make myself a more competitive applicant aside from the basics like doing research and keeping a high GPA.

I've heard people recommend applying to scholarships (I know about the Goldwater scholarship, which is pretty competitive, but is there some resource to find scholarships to apply to?), REUs (I know about Amgen scholars but not sure how to be a good applicant for those), going to conferences (once again, not sure if there are like well-known conferences or even how to find conferences), participating in clubs related to my major, TAing, and applying for Fellowships the year before I start a PhD. If anyone wanted to add on other things to do or give advice on how to pursue these opportunities, I would really appreciate it.

If it matters, I've already done ~1.5 years of research at my previous institution (will be a coauthor when they finish our research and publish), and am contacting professors right now so I can start in a lab when I'm on campus this Fall. I've also considered taking an extra year to graduate (might not even be a choice depending on what courses transfer over) because it would spread out my courseload so I could focus more on lab and focus on other opportunities, in addition, I think research funding would be more available or at least anticipated to be then. My only concern is that taking an extra year might hurt my application if my only reasons are what I said above.