There is tons of great advice in this thread from a lot of folks with a wealth of knowledge and experience in college admissions. To the extent it is at all helpful to those who will be tackling this process in the next year, I thought I'd put down my thoughts based on our experience and what we learned. This is obviously just one person's perspective based upon an clearly limited experience (and I'm sure there are lots of contrary opinions), so take everything below with a grain of salt.
1) Testing - To the extent testing remains relevant for admissions, I recommend that kids take both the SAT and the ACT and do so early in their Junior year. Although there is a great deal of overlap in terms of substance, the testing structure is actually quite different between the two. For many kids, one test will play to their skills and abilities more than the other, and it's often hard to know which is which until you take it. Once that has been determined, they can focus on the test that is the better fit for subsequent sittings. And, who knows, your kid might get lucky and knock out their goal score during their junior year - which means one less thing to worry about over the summer or in the fall.
2) Writing Section - Relatively few colleges take the writing section of the ACT into account (may be the same for SAT). My recommendation (for what it's worth) is to not sign up for the ACT + writing test unless you plan to apply to a school that requires it (UC schools, and a number of the elite schools). Why do I say this? Well, my kid was able to get a really good ACT score. But he completely bombed the writing section. And while none of the schools to which he applied required the writing section (and even stated expressly that they didn't consider it), there it was on every single score report submitted to every single school. The proverbial fly in the ointment. This created a lot of unnecessary stress. If you do decide to do the optional writing section, don't blow off preparation for it (like we did). Be sure to allocate sufficient time to understand how it is graded and do a bunch of practice essays. This requires a fair amount of work (and your kid will already have a ton of work), so if none of the schools to which your kid is applying take it into consideration, it's best just to pass in my opinion.
3) SAT Subject Tests - These suck. But they may be helpful if you're applying to highly competitive schools. Others' experience may differ, but these took a lot of effort in terms of preparation and are super hard. For example, my son took the Math II and Physics Subject Tests - the Physics test covered topics that are not covered in AP Physics. So my son had to learn completely new subject matter areas that he'd never seen before (these subjects are now being covered in his AP Physics C class, but that is long after the SAT Subject test is required to be taken). And if you don't score well on those tests, it's really not worth submitting them. In the end, my son did "okay", but the scores weren't high enough to help his application so we didn't submit them. In hindsight, that was a lot of time and effort wasted. So if you are applying to schools where these are required and good scores are necessary, get ready to spend a lot of time on preparation (at least for the Math II and Physics tests - not sure about the others).
4) Campus Visits - To the extent you can, start these early (sophomore year even), and don't limit yourself to schools that your kids are interested in. In fact, it's both inexpensive and informative to go visit local campuses (within driving distance), even if it's not for a formal tour. By spending some time in different campus environments, your kid will likely start to develop some opinions on what they are looking for. Large school versus small. Urban versus non-urban environment. Old historic buildings versus modern spaces. Some of these criteria may seem irrelevant or arbitrary, but they can also be extremely helpful when it comes time to actually narrow down the list of schools to which your kid will be applying.
5) College Selection - Think outside the box. Whether its doing independent research, talking to your school counselor, or even engaging a consultant, try to keep an open mind. There are so many great schools out there, and the one that is a great fit for your kid may be a school you haven't even heard of. Don't fall into the trap of applying to the same list of schools that your kids peers talk about, or your own personal list. We had someone help us come up with a target list based on the things my son was looking for, and there were schools that ended up on the list that we would have never considered because our knowledge was simply too limited.
6) Essay - Personal Statement - Do your best to butt out. Let your kid find his or her voice in the personal statement. Many schools have counselors, or even optional classes, that help kids brainstorm about topics for their personal statement and what is likely to be impactful with colleges. To the extent possible, let these third parties guide your kid through the personal statement process. Because, as a parent, you're too invested and too close to the situation. For me, I knew exactly what my son should write about in his personal statement, but I knew that he would immediately resist any advice that I had to give him. What I learned later, though after taking a completely hands off approach to the process, is that I was wrong. Because when I ultimately read his personal statement (which wasn't until it was in final form), I realized that his voice was altogether different than my perception of what his voice should be. Was his personal statement a masterpiece of inspirational prose? No. But after reading it, I felt I knew him better (and I already know him really well!). Ultimately, I came to realize that the goal of the personal statement is not to impress the admissions department. Rather, it is to show them who you are. And a parent being overly involved in that process will hinder the achievement of that end (in my opinion).
7) All the Other Essays - There are likely to be a ton of other essays your kids will have to write. To the extent you can identify the schools to which your kid will be applying, work on these over the summer. Fall semester of senior year will be very busy, and the last thing your kid needs to slog through is writing 30 different essays. Also, for the "why our school" essays, your kids need to do their research, and it can't be superficial. They need to talk specifics. Whether it's particular research opportunities they're interested in, certain clubs they want to join, specific aspects of campus life, it needs to be detailed and specific to the school.
8) Extracurriculars - I don't have a strong opinion on this topic, because I feel it's all a crap shoot and schools can take very different approaches in terms of what they take into account. To the extent that I have any opinion, I think demonstrating a commitment (extended involvement, leadership, etc.) to fewer activities is better than a scattershot check the box involvement in a bunch of different things (this applies to service as well). Unfortunately, not all applications allow you the opportunity to explain the depth of your commitment to a particular activity, but if they do, be sure to take it.
I'm sure there are other things, but that's what I have off the top of my head. Best of luck to the class of 2021 and beyond as you embark on this process.