Traveling should be fun no matter where you go to. When visiting a different country that has a different culture than the one you're used to, it is easy to make a mistake and insult the locals. You should know that words can be more offensive than gestures, so be prepared and learn a few things you shouldn't say out loud when you're traveling abroad.
Bragging about your family's wealth
This one shouldn't even be mentioned, but there are plenty of youngsters making this mistake. This usually is true for those coming from wealthier families and start saying how will they use their parents' credit card to pay for stuff. Talking about such things will not only make the locals not like you, and think you're spoiled, but can put you in a dangerous situation. Experiencing a rubbery in a foreign country is not even the worst that can happen.
Comparing the destination country and your home country
Comparing your own country and customs to those of the country you visit is not bad per se. However most tourists feel the need to imply that the way things are done back home is better. As a result they complain a lot and show dissatisfaction expecting the locals to immediately change their customs and habits. Tourists need to understand that this is just a difference and doesn't mean something is better or worse than what they're used to.
On a side and related note, you should also refrain on commenting how the place you're visiting was better a decade or two ago. The other folks accompanying you will imagine you didn't travel much or that you don't like them. If you decided on your current destination because of some online review you could mention this, but keep in mind everyone has different preferences and opinions.
These bad jokes can be related to names of places, to what the foods look like, and so on. Additionally, jokes that make use of people's skin color or race should also be left back home because you will never know who hears.
Constantly asking if you reached the destination
Inquiring about this can only she bad light on you. The others will think you're bored, that you don't enjoy the trip, and even that you found nothing better to do with your time or money. If you really got bored, try and nap till you reach the location, especially if you don't have to walk.
Mentioning out loud you hate the food
You should know that each country has its own cuisine that is more or less popular around the world. You should be considerate towards those preparing the food as it might be hard to come by or prepare. Everyone will think you're rude spoiled, and ignorant. They will also wonder why are you traveling if you are not willing to try out new weird things. You should instead first try having a bite or two, and then politely explain you don't think you can finish your serving.