Here are The Traveling Professor's tips on how to foolishly spend money on an international trip:
Stay at a Poorly Located Hotel: Staying too far from where all the action is will cost time and money to get where you want to go.
Paying for Museum/Attraction Admission at the Museum/Attraction: You'll pay the highest price possible by walking up to the ticket office and there is probably no way to skip the long lines. For example, we always buy river cruises, train tickets, museum passes, etc. in advance. The price is usually better and depending on what we are buying tickets for, we might bet preferred seating, expedited admission, etc.
Eating at the Touristy Spots: If you ever had coffee on St. Mark's Square in Venice, you know what I am talking about. Step a few blocks off the beaten path for a better meal at a better price.
Paying for Bus/Train Tickets as You Go: For instance, a bus/metro ticket in Paris is about 2 euros per ride if buying it on the spot. Buy a weekly or daily transportation pass makes more sense.
Buying the Cheapest Possible Airfare: Now you've got to pay for seats, baggage, meals and just about everything else.
Over-Tipping: In the U.S. and Canada, tips of 15%-20% are pretty much standard. Not in Europe. I have actually had taxi drivers in Iceland refuse to accept tips from me. For just about everything, meals, taxis, tour guides, etc. a 5%-10% tip is plenty.