En espanol | We all know to lock our hotel doors and avoid flashing our cash when we travel. But there are some critical facts many people don't know, and they can make the difference in whether your next trip is happy or horrible.

See also: How to upgrade your next vacation for free!

Hotel room safes? Not as safe as you might think. — Photo by Roussell Photography/Alamy

1. Airplanes aren't as clean as you think.
Thirsty on your flight? Stick with bottled water. Airplane water has been under review by the EPA for traces of E. coli for six years.The holding tank on planes is seldom cleaned, so pass on the coffee, tea and ice cubes, as well. And planes sometimes refill at foreign airports, where water standards can be questionable.

Airplanes aren't thoroughly cleaned until they reach their final stop for the day, so you might be stepping into three flights' worth of collective grime. Have a late flight? You better bring disinfecting wipes to clean down the tray table. Avoid putting belongings in seat pockets and using airplane pillows and blankets. Finally, don't wash your hands in the airplane bathroom, which uses water from the holding tank. Use hand sanitizer instead.

2. That hotel room safe … isn't.
Did you know hotel employees usually have a master code to access the room safe in case a guest loses the key? Keep your passport and other valuables with you, or store them in the hotel's main safe or in your suitcase instead. Pick up a good cable lock — like this one from Pacsafe — and fasten it to something immobile when you leave your room.

3. Most credit card rewards are nonrefundable.
Are you booking rewards travel with your Capital One, Chase or American Express points? Better be absolutely certain about your travel plans. Most earned points that are not part of a major airline frequent-flier mile program can't be redeposited, so if you have to cancel your reservation, you won't be getting them back. Travel insurance providers often don't insure awards travel, so don't count on them to rescue you, either. 

4. RV rentals can have hidden fees.
By now you have seen those commercials: A Rockwell-esque family of four and their retriever pile into a Winnebago and hit the open road. But before you try RV-ing yourself, realize the rental companies may ask a $1,000-plus security deposit. Be extremely careful about children, pets and smoking (do we really have to mention that last one?) in the vehicle. And bring your own towels, linens and kitchenware or expect to pay fees from $50-100 to rent them.

5. Cell phones on cruise ships will drown you with fees.
Surprised that you get reception on the open water? You'll be even more surprised when the bill arrives from your wireless company. Don't be fooled because you're close to the coast. If you're connecting to the ship's onboard cell tower, you'll pay hefty roaming fees that could be up to $5 per minute. Wait until you get to shore and find an Internet café where you can use a service such as Skype.

Next: Should you buy rental car insurance?