MaeN

The best thing a potential advisor said to me at a visit was "No matter what you decide, when you do, you'll convince youself it was the right choice - so it really doesn't matter!"
Obviously it DOES matter and this decision is going to determine a lot about your life but honestly I thought that was kind of comforting? I haven't made an official decision or signed anything, but I know what school I'd like to go to. And as long as I get off the plane for my visit and it isn't a total nuclear wasteland and as long as my potential department doesn't throw rocks at me, I'm kind of deadset on going. 
And it terrified me. For like 3 days I couldn't sleep because I was saying goodbye to all the potential lives I'd lead at other schools. And after that brief mourning period (it honestly kind of felt like the 5 stages of grief) - I have just been so hype about the school I'm pretty set on. I've been researching apartments. I've been looking at cool places to eat in the city. I've been looking at the faculty and kind of falling in love with it. 
So long story short: mourn the life you could have lived at another school. Let it go and let you in an alternate universe enjoy that life and then just get HYPE about your new school. This change is terrifying but it's so exciting! 

I'm deciding between two schools and I'm definitely leaning towards one of them, but I don't feel ready to make the decision. I really like both schools and advisors; each has different pros and cons. I know that I'll be missing opportunities wherever I don't end up going, just like with any decision. So, once you start making up your mind about which school to go to, how do you decide that you're ready to decide?

At the end of the day whatever you do, you're choosing one option and letting go of all others. That's unavoidable. If you've gotten feedback from trusted advisors and you've talked to everyone yourself, there's not much more to do than take the plunge. I usually like to spend a day or two pretending as if I've made up my mind and getting myself excited about the next steps (new city, new program, moving there and apartment hunting, new people) before making it final. But then that's all that's left. You'll never know what's down the road not taken, such is life, but all you can do is make the best decision given information you have available. Do that and trust yourself. 

@sprklinthe I get that this can be a sore subject, but that's actually why this thread was made. I didn't get in anywhere last year and it hurt. Luckily, there are multiple places to discuss that (and the worry when waiting) on this forum. This thread was specifically created so that there would be a place for those with admissions to let off stress without making those still waiting feel bad. 

Best of luck with your programs! Hopefully, you'll be joining us in worrying over the decision process soon enough! <3 

Pick only weekdays. Best to pick a day adjacent to a weekend so you can use that as a travel day (and/or a day to explore the city on your own, if you want) since that minimizes classes you miss. If you visit on a weekend, there will be no one around. Keep in mind that these visits require a lot of admin support and they definitely do not work on weekends. Professors might do some email or work at home on a weekend but they won't be in the building. Also everything will likely be locked. 
It's tough to miss classes but ultimately, you will learn a lot more and get a lot more out of these visits than a class! I would recommend choosing a Thursday and Friday visit. Use Wednesday to travel to the school, visit and Thurs and Friday, then pick a later flight home on Saturday (or Sunday morning) if possible so that you can explore on your own if you wish. Also, choosing to stay for a Friday evening usually makes it more likely that students will do social things with you that night (compared to a Wednesday night or something). But this depends on the culture of the department. For my old department, grad student visits are a huge social event and there's a small committee of students that form to create a fun experience for visitors!
Another strategy is to check the department website and see if there are major events happening on certain days. For example, when is the weekly department seminar? Or, is there a seminar series where grad students present? Attend those days. It gives you a very good feel of what it would be like to be part of that department. Personally, I would pick Thurs/Friday over seminar days because the social events and the interactions with students was a high priority for me in choosing schools since this is a time where you can informally interact with students and get candid and honest answers. However, if that doesn't work, then the seminar days are good too. The caveat is that it's a small sample....all seminar series have hits and misses so if you go on an abnormal week, you won't really know if it's the same every week. But it's better than going on a day without anything.
Finally, your sidebar info says "Planetary Science" and I think the nature of the visit could really depend on the department. I did my PhD in a Planetary Science program that was part of a bigger geological/planetary department rather than an astro/physics one. I noticed that geo/earth science departments tend to have more community events in their visit dates with grad students doing a large part of the planning and there were things for me to do all the way until bedtime, while my astro/physics visits were just the "business hours" and the students might take me to dinner but then that was it. Could just be my limited experience though. Also, one of the geo/earth science departments included a field trip in their visit dates! Might not happen if you're visiting by yourself though.

Thank you! That's really helpful, especially thinking what I want to get out of it.

I'm applying for Fall 2018 and I was wondering what people typically do the summer between undergrad and grad school. 
Most REUs (Research Experience for Undergraduates) and my school's research opportunities don't let graduating seniors apply, but there are still internships and a few other academic jobs I can apply for. There are non-academic jobs, too, of course. Depending on where I end up, I may also be able to start rad school earlier in the summer. I definitely want to be doing something, but I'm not sure what.
Any suggestions or perspectives on this?

I'm all for relaxing, especially in the summer, but I'd suggest paid employment.  It doesn't have to be full time, but graduate school is filled with unexpected expenses and it wouldn't hurt to have a little extra money put away.  If your summer gig is also something fun then that's a plus.  If you have the opportunity to travel (even if just to visit relatives or friends you haven't seen in awhile) that could also be fun.

I would also recommend that you take the summer off, if you can. You can even move to your new place early (1 month or so) if you want more time to settle in to your new home before school begins.
However, some people either want to be doing work or need to be doing work (for income). In that case, many programs in our field do allow an earlier start! At my old planetary science program, about 1 in 4 or 5 new students will start early. You can also split the difference...take a few months off then start a few weeks or 1 month early or something.
Finally, although most REUs will only hire continuing students, your undergrad thesis advisor may be willing to hire you to finish up any loose ends after you graduate. Or, you might be able to find some other research arrangement outside of these formal REU or REU-like programs. Between my BSc and MSc, this was what I did (well 2 months of part time research + 2months off). Between my MSc and PhD, I only had 1 month off to move because I finished my MSc thesis, defended it, submitted corrections etc. just a few weeks before my PhD program started.