© Kinga Howard
The flight was a bargain. The hotel was a steal. And then you get home, look at the statement, and find that a trip you budgeted at $900 came in somewhere north of $1,400 with no single line item to blame. It feels like this happens to us on almost every other vacation, and if you dig a bit, you might find that the pattern turns out to be fairly consistent, and almost none of it was the fun stuff.

Leaving transport until you arrive

This is the big one. Sorting out how you will rent a car on vacation while you are still at home, rather than at a desk in an arrivals hall, is usually the difference between a sensible daily rate and whatever the counter feels like charging a tired person with luggage at 10 pm.
Airport locations in particular reward booking early, because the vehicles that go first are the small, cheap, sensible ones, and what remains by the afternoon of a busy Friday is a large SUV at a price designed for people with no alternative.
Booking ahead also gives you the chance to actually read what is included. What the mileage terms are, whether the insurance you already have through your card makes the counter-upsell unnecessary. Ten minutes at your kitchen table is worth about $200, which is a better hourly rate than most of us earn.

Taking the flight that lands at one in the morning

The cheap flight is genuinely cheaper right up until the moment you need to get somewhere at 1:40 am, when your options narrow to an expensive ride or a hotel night you did not plan to pay for.
What good is saving $60 on the fare if you wind up spending $80 on a taxi because nothing else was running, and then sleeping badly enough that you write off most of the next day? Check the arrival time before the price. If it lands after 11 pm or before 6 am, the fare has to be dramatically better to be worth it.

Saying yes to "would you like to pay in your home currency?"

The card machine abroad will offer this, and it always sounds helpful. It is not. That is dynamic currency conversion, and the exchange rate baked into it is generally worse than what your own bank would give you, sometimes by three or four percent.
Always choose the local currency. Every time, at every terminal, in every shop. It takes one extra tap, and over a two-week trip, it adds up to a decent dinner.
The related mistake is airport currency exchange, which has the worst rates anywhere in the country. If you need cash, take a small amount out of a bank ATM once you are properly in the city.

Buying your entire routine twice

There is a specific type of spending that happens on day one of every trip, where you stand in an unfamiliar pharmacy buying full-sized versions of things you already own because you left yours at home or decanted them badly.
Sunscreen is the classic. So is conditioner, which nobody remembers because hotels provide something they insist is conditioner. Toothpaste, contact solution, painkillers, a phone cable.
A proper travel kit that lives permanently in your suitcase, restocked when you get home rather than the night before you leave, kills most of this. Investing a little in decanting your skincare into travel sizes properly, with bottles that actually seal, pays for itself in about two trips and saves you the pharmacy tour on your first morning.

Not reading what the hotel rate includes

Resort fees, parking, breakfast, Wi-Fi, the charge for the safe you did not use. Some of these are disclosed at the point of booking, and some appear on the bill at checkout, and the difference between an advertised rate and a final rate can be twenty-five percent in certain cities.
Look at the total for the whole stay, not the nightly headline. And check the parking situation specifically if you are driving, because downtown hotels frequently charge more per night to park than some apartments charge to sleep.

Booking the tourist thing you could have walked to

There is a category of paid experience that exists mostly because visitors do not know the geography yet. The bus tour to the viewpoint that is a twenty-minute walk uphill. The airport transfer that costs four times as much as the train. The guided walk past things you can see for free.
Some tours are excellent and worth every cent, particularly anything with genuine expertise attached. The test is whether you are paying for knowledge or paying for transport you could have arranged yourself for a fraction of the price.

Souvenirs bought on the last afternoon

Panic shopping is expensive shopping. The final-day scramble through an airport gift shop produces things nobody wants at prices nobody would pay in a calmer moment.
If you want to bring things home, buy them as you go, when you see something you actually like. And be honest about the baggage cost, since a checked bag on the return leg can easily exceed the value of what you are carrying in it.

The habit that fixes most of this

Before you book anything, write down the four numbers that will decide the trip: flights, accommodation, getting around, and a realistic daily spend for food and activities. Not an optimistic one. The number you actually spend.
Total it. If that total is uncomfortable, change the trip now, while changing it is free. Most of the money that disappears on a vacation does so because it was never assigned anywhere in the first place, and the decisions that would have saved it all had to be made weeks before departure.