You have decided to visit some city (let it be the big city or some small rural city), but you only have one day. Well, that's fine, that happens. Even if you have one day you can make the most out of it by planning your trip properly.
But be prepared to make most of your day because of that plan, otherwise, you'll end up visiting fewer places during the day and that is going to feel like your day is incomplete.
Start your day early
That means going out of the door around 7 or 8, earlier the better, but waking up too early might make your trip less worth it (because you'll get mid-day fatigue). So, better to start it up reasonably early.
Try to minimize eating out
I'm personally a minimalist kind of person who doesn't believe that going to restaurants will make your experience better. In fact, I've traveled a lot and in my experience going to restaurants mostly just waste your time and money unreasonably. Just imagine: if you go to a restaurant a wait time might be around 20 minutes - 40 minutes (might be even more if you order something exotic or there are too many people).
Let's say on average it's going to be 20 minutes for them just to prepare the food. Now add the fact that it's going to take at least 10-20 minutes to consume the food. That's about 1 hour of the time wasted right away. Now multiply this three times and now you have it: 3 hours of your day are gone. Now, let's talk about expenses. On average, eating in a restaurant will cost 15$ for a person per meal. 3 times a day: 45$ just for 1 person. Is it worth it? I don't think so, you might do something more important during that time. Just make your own sandwiches/salads, snacks, or go to some fast food chain as the last option. Although, I suggest picking up the healthiest options there.
Be flexible on your travel plans
If you travel alone or with someone, be prepared to make some changes on a fly to your plans if something happens. Let's say you go to a national park and it's closed or you are late to a museum and it's already close - don't panic or get disappointed, you can always find out another plan or a way to spend this time. Just be that person who is willing to be flexible, it will help in a long run and save your nerves. Don't argue, ever. If you have limited time and travel with someone, very likely there will be a room for you to argue. Don't let that happen, find a way to compromise. You just have one day and spoiling it with pointless conversations will not make it any better, in fact, it is going to destroy the whole point of your trip.
Have a list of the places to visit
Now we've come to that point. In order to have a nice road trip, you need to have a list of places to visit before your travel. You don't want to go to one place and then realize that's the end of your plan, it's 1 pm and you have plenty of time, but you don't know where to go. Just have some, even approximate plan of places that you want to visit.
Calculate how much time each place is going to take approximately. Let me say honestly. Most of the time we assume that it's going to be enough time for us to visit someplace and later on we realize that the place is so interesting that we want to spend the entire day there. Be realistic, if you have just a day most likely you can have 3-5 places visited at most. Make sure that you take a travel time between places into account.
Don't go with somebody who doesn't share your views on travel. That might be debatable, but let's think more about this. Imagine, you are a minimalist, who only believes that nature and hiking are the best way to spend the time. Now, imagine going with a person who loves food and nightclubs. Will you find a compromise? Well... It's hard to predict because sometimes people are going to say that they are fine with whatever plan you come up with, but later on, you both realize that it's not going to work. So, try to predict if it's going to work out before the trip. Remember, you have very limited time.
Don't push yourself too much
If you know that can only physically cover 3 places don't try to visit 6 places. Yes, the point is to be as efficient as possible, but what is the point of visiting too many places if you know that you are so tired, that you won't even enjoy it? I've been into those situations myself at times and I while I was doing fine, I felt like I was going like crazy, missing the current moment.
Buy multi-attraction passes to save some money. This is a general rule for travel even for multiple days, but if you have a single day buying a pass will allow you to prepare even better for your short trip. And the important thing is that you'll not feel bad about paying 30$ for a single attraction because you have bought a 60$ pass which covers all of your attractions. Simple, right? Everything is simple if planned a little bit ahead.
Plan ahead for a place to stay
When we are talking about traveling to big cities especially, you may realize that finding a place to stay is not that easy and it might take your time and cost more at the end if you book it at the last moment. To avoid surprises, I recommend booking a place to stay in advance. For example, if you are thinking about a house or a room rental I would recommend you trying out Airbnb. Sign up and look up the option that you like the most. There should be something that you are going to love.