I (male, middle-aged (ugh), American) just returned from a 17-day solo trip to Rome, Budapest and Prague and thought I would share my list of "don't"s.
Nothing Earth-shattering here (and nothing city-specific), but maybe some Redditors will find something useful. (And they're not really hard and fast don'ts, it's just how I structured the post. By all means, do what works best for you.)
* Don't take taxis/Uber/Lyft everywhere. I took transit (buses, subways, trams) much more on this trip than ever before and it was great. Add in a lot of walking and a few scooters and I experienced the cities in a way that isn't possible when riding in a car from the hotel to Site A to Site B and back. As a bonus, I spent less money and really didn't sacrifice much in terms of time or convenience.
* Don't dress like a stereotypical American. Again, this was a new one for me. I've never been the most stylish person (at home or abroad), but I tried to improve my look a bit on this trip, especially while in Italy. Nothing crazy - for example, think a button-down linen shirt instead of a t-shirt with a sports team logo - but I actually liked it. As with riding transit, I felt like I was experiencing the city in a somewhat deeper way, and I felt a little less like a tourist. Interestingly (to combine these first two bullets), I was asked for transit directions several times (at least once in the local language), and was actually able to provide useful replies. I felt like an honorary local! (Granted, they were pretty easy questions - like, does this line go to Roma Termini, which happened to be my destination - but still.)
* Don't use the hotel gym. Yes, the hotel gym has its advantages - especially if, against all odds, it is well equipped: There's no extra cost, the time commitment is minimized, and maybe there will be a bowl with free apples. But - to continue a theme - there is great value in doing the non-touristy thing. When I go to a local gym, it often takes me to a part of town I wouldn't otherwise see, it typically puts me among locals (instead of other hotel guests), and, once again, it just enhances my feel for the city. (And I can still pick up a couple of apples when I get back.)
* Don't sit at a table or booth in a restaurant/bar. Sometimes there is no choice, of course, But if the location has a bar area, I prefer to sit there. The opportunities for interaction are much greater.
* Don't be phone obsessed. I like to go out to bars and pubs in the evening and the siren's call of the smartphone is even greater than usual when sitting there by myself. But watching videos and reading articles is not particularly sociable or welcoming - I could do that in my hotel room, after all - so I make an effort to put the phone in my pocket and present myself in a way that conveys interest, curiosity, respect and friendliness. Yes, I still look at it at times, but at least I'm giving interaction a chance.
* Don't be ratings-obsessed. I don't know how many times I've looked for a place to eat or drink and found a dozen options around me that were all 4.4 to 4.6 on Google Maps. So, this time, I tried to just go with my gut more often while walking around and select places that, for whatever reason, caught my eye. It worked well. No bad experiences (or bad food), I found myself in places that felt truly local at times, and it felt good to not be told where to go by an app.
* Don't completely avoid a foreign language. I am embarrassed that I only speak English and grateful that the language is so widespread. Even in cities where English is commonly spoken, though, I have found it useful to know at least the following eight words/phrases in the local language: Hello, goodbye, yes, no, please, thank you, you're welcome, pardon me. (As an introvert, that actually covers a large percentage of my conversations.)
Thanks for your time. I look forward to reading much better advice in the comments.