The stuff that actually derails a trip isn't usually the big dramatic thing, it's the boring logistics nobody double-checks. Here's what I lock down before I book anything else.
Different countries have different entry rules, and those rules can change. Visa requirements, arrival forms, travel authorizations, entry fees, passport rules, all of it varies. Check the current requirements before you leave, because “I didn't know” is a terrible reason to spend the first part of your trip stuck at the airport.
When I went to Toronto, I chose Billy Bishop because it made the most sense for where I was staying. It's on an island, which already made it more interesting to me, and they even have a water taxi option. I used the quick pedestrian tunnel instead, came out downtown, and walked about 20 minutes to my Airbnb. If I had flown into the other airport, getting to my stay would've been a much bigger production.
I don't need every minute of arrival day planned, but I do want to know how I'm getting from the airport to where I'm staying. Train, rideshare, shuttle, taxi, private transfer, whatever makes sense. The goal is simple: don't land tired, hungry, and confused only to realize you have no idea how to get out of the airport.
Before I book an excursion through a third-party site, I try to find the actual company running it. Sometimes you save money, and sometimes the company keeps more of what you pay because there's no middleman taking a cut. That matters, especially when you're booking with local operators. And yes, you BETTER be supporting the locals. 😡
I'm planning Peru right now, and Machu Picchu is exactly the kind of experience I would never leave until the last minute. Tours, entry circuits, trains, and certain passes can sell out well ahead of time. I like leaving room in a trip for spontaneity, but if something is one of the main reasons I'm going and access is limited, I lock it in early.
A pre-downloaded map is one of the easiest things to have ready when you're traveling solo. It gives you navigation if service gets spotty, but I also use it as a running list of places I want to try. Restaurants, shops, bars, landmarks, whatever catches my eye gets saved so I'm not digging through screenshots later.