I did a lot of thinking during the trip that Melissa Moffet and I took to Scotland, and here are my thoughts wrapped up in a few paragraphs.
I think no one would dispute that international traveling can be stressful. Starting with jet lag, to getting where you need to go, different currency, phone service (or lack of), to oftentimes a different primary language - it can all be a bit overwhelming. However, there are a few things you can do in advance to help mitigate some of the stress. This is what I do every time when I travel for myself:
Download an offline map of destination. I do this in Google maps, not sure if Apple maps has the same functionality. But it allows you to have a map even if you have no phone service.
See if your mobile carrier has an international plan and if it will be worth it to you to have data for your trip. I have Sprint and they have a free international plan add-on where 2G data and texting is free. 2G is really not sufficient for video streaming but it works fine for messaging.
Pack your ATM card. Cash advances through credit cards are expensive.
Make sure your passport has over 6 months before expiration.
First night accommodation planned out: I find this to be a huge stress reliever. Even if the rest of your trip is unplanned, coming off a red eye and knowing where you will be sleeping the first night is a mental relief. If it's an airbnb, I ask the host the best way to get there.
Figure out how you will get around. Taxi, Uber, walking, public transport, rental car? For public transport, how does ticketing work? I read that the easiest payment method for the London Tube is cashless contact using Google/Apple Pay, so I set that up before leaving the US. If you get a rental car, who will drive? Would a tour bus be better? Consider and discuss these questions with your travel buddy.
With logistics out of the way, the next thing is to consider your travel buddy. With the stresses of travel and the number of things that can go wrong, it's really important you and your friend(s) can balance each other out. It does no good to have everyone freaking out if/when plans go awry. Here are some traits that I find make a smoother trip:
Somewhat same sleep schedule. If you are driving a full day and you need help navigating but your friend is passed out in the passenger seat because they have been partying all night, that makes for a grumpy scenario. Melissa and I both are early risers and early to bed, so that worked out well.
Similar interests and energy levels. The only time I left Melissa was on the hike up Old Man Storr. I am not a hiker, especially a hiker in the cold drizzle and wind. But if one person is outdoorsy and the other is a museum goer, you may have to balance expectations on what to do together. Or if one person likes to run around for 12 hours and the other needs more rest time, this will also need to be accounted for.
Be respectful of space. If you needs morning quiet, meditation, journaling before starting the day, then set that requirement early on so it's not an issue.
Be able to have a normal conversation with them. Long car rides equal conversations. Make sure you can talk to this friend about tons of different things, including pooping. Because, you know, that always comes up.