Nobody loves vacations and travel more than I do. Back in high school and college, I spent two summers working as a tour guide at a historic site and one summer in a national park gift shop. It was glorious for the most part, but it did teach me an important lesson: something about venturing outside of your daily routine, even when it's supposed to be fun, can bring out the worst in people. (Ever been to a Disney park? Around 3 p.m., take an informal tally of the number of crying kids and compare it to the number of happy kids you observe. To say nothing for the adults.)
This lesson has been reinforced over and over in my adulthood as I've planned and taken my own vacations (and been a dick myself more than once, I'm sure).
Vacation dickitude can happen on safari or along a three-month backpacking tour of Europe just as easily as it can happen at a national park or a weatherbeaten family resort in the Poconos, and the dickitude radius increases the further you get from home - it's possible to be a dick not just to your own friends and family, but to public-facing employees, your fellow tourists, and total strangers.
Respect the local culture and history (or yes, you can be an Ugly American even if you're in America!).
Not everyone has to be that research-happy overplanner whose knowledge outpaces the tour guide's, but it is good to at least have, or acquire, some sense of what happened at the site you're visiting. If your fellow tourists are feeling some sort of emotional impact as a result of visiting a historic site, maybe keep the gallows humor to a minimum (or at least a minimum volume) and maybe don't take a grinning selfie in front of a place where several thousand people died.
Also, there's nothing overtly wrong with visiting sites that are spiritually significant to people other than you. Visiting churches, synagogues, and other sacred sites can be deeply meaningful even to people who don't subscribe to the religion at hand. But when a fellow visitor really IS observant, and they're trying to get in touch with their God, it's good non-dickish form to avoid disrupting their meditation by talking loudly, taking lots of flash pictures, or otherwise getting in their way.
Since we're talking religion, let's also talk politics. If the general political atmosphere or other beliefs of a particular region or attraction don't resonate with your own, that's probably not going to change in any meaningful way if you bring it up to locals, employees, bystanders, or your own travel companions at every opportunity.
Before you get up on your soapbox, regardless of whether you're objectively right or wrong, think about this - what's it going to accomplish to tell everyone within earshot about what a giant jerk Thomas Edison was in real life, or that zoos are just prisons for animals, or that the planetarium is purest fiction since the sun actually revolves around the Earth? It's just going to irritate others and get you in a lather.
If you know a particular site is going to be brutally incompatible with your beliefs, maybe just don't visit it? Besides, you're on vacation - what's relaxing about crusading?
Recognize that the rules apply to you.
Look, nobody likes rules. You may not agree with all of them, but obeying the rules is part of the tradeoff for getting to experience the whatever-it-is. If the museum doesn't want you to take photos, don't take photos. If the restaurant kitchen closes at 9, don't demand food at 9:05. If you're not supposed to stand up while the ride is in motion, don't do that shit.
The rules are for everyone, they're usually there for the good of everyone, and the benefits of breaking them and/or getting special snowflake treatment are usually negligible at best anyway. Failing to obtain that glarey digital photo of Mary Todd Lincoln's teapot will not diminish your enjoyment of your visit, I promise.
During my tenure as a national park gift shop clerk, I once spent ten minutes patiently and repeatedly explaining to a customer that I could not grant her access to our employee restroom, which was a dingy little closet in the back of our basement stockroom. Was there a medical issue, major emergency, or other reason she couldn't use the public toilets? No, not really. Her 7-year-old just didn't feel like waiting in a short line.
In the time she spent trying to circumvent the rules, she and her kid could have gone to the bathroom about nine times each. And the public bathrooms were way nicer than ours anyway.
Acknowledge that there are people around you who aren't on vacation.
I work in midtown Manhattan, right around the corner from at least a dozen big hotels. Naturally, I end up inadvertently crashing someone's vacation at least a few times a day on my way to and from the subway.
Tourism is huge in New York City, and I definitely don't begrudge anyone who wants to come see it. It's awesome here. That's why I live here. But my life isn't a nonstop vacation party. Some days I just want to escape my cubicle in a timely fashion so I can cook dinner, hit the gym, fall asleep in front of Conan, and do it all over again tomorrow. On a day to day basis, the employee dorms in Estes Park, Colorado, weren't all that exciting either (although I never did get tired of the scenery).
What I'm trying to say is this: don't allow your enjoyment of the sights to interfere with others' ability to go about their daily routines in and around those sights. For the love of all that is sacred and holy, do not stop in the middle of a sidewalk or at the top of a subway staircase to check your map, do not pause amid a moving crowd to crane your neck up at a pretty building, and definitely do not walk five abreast across a busy sidewalk in the middle of rush hour.
Don't take your stress out on the people serving you.
Your vacation-related misery is most likely the result of the weather, the traffic, whoever designed the place, whoever fixed the ticket prices, your GPS, your own overtired children, someone else's overtired children, the 80,000 other people who decided to vacation to the same place you did, or some combination of the above.
Nine times out of ten, it is not the immediate fault of the kid making minimum wage whom you are currently confronting. Yelling at them might make you feel better in the moment, but not only will it make the employee feel like shit, it probably won't go very far to fix your problems.
Most public-facing employees, be they gift shop attendants, ride operators, tour guides, or ice cream scoopers, are not empowered to do very much for you in the moment. So if your particular complaint is more important than, say, someone puked in the t-shirt aisle, it's best to politely raise your concern and ask for a manager. If your problem goes deeper, it's probably best to get in touch with a higher office after the fact. Most companies welcome constructive criticism, and who doesn't love penning a sternly worded letter to corporate?
Also, if you had a great time and great service someplace at the hands of a specific individual, tell their managers, Yelp, TripAdvisor, and anyone else who might care. At the bare minimum, positive feedback cancels out a couple of instances of being yelled at in the mind of some beleaguered employee.
Basically, not being a dick on vacation is pretty much the same as not being a dick anywhere else, TO anyone else – it's eschewing entitlement, exercising common sense, and having a modicum of self-awareness and concern for others.