I am not saying you can not eat out when you travel. This also might sound like a huge contradiction but I try to teach people to be as perfect as possible when they are at home. By this, I mean exercising and eating exactly how they should in order to achieve whatever goals they are working towards. If they are able to do this for 50 weeks out of the year, then I think that when they spend their 2 weeks of vacation, they should be able to do whatever and eat whatever they want. They don't even have to exercise if they don't want to. This is all under the condition that when they come home they are right back into their routine. Doing this will ensure them to stay on track for the long term and it really shows that they have made the change to a healthy lifestyle.
This is not realistic for everybody but this is at least this something to strive for. This also does not not apply to people that travel more than two or three weeks a year. If you travel more often than this or if you are not exactly perfect when you are at home, this is when the planning comes into play.
Traveling for a short trip whether it is for vacation or for work can be handled in very similar ways. The first step to planning is to pack food for the time it takes to get to your destination. Choose healthy items that travel well like sandwiches without condiments, fruit and meal replacement bars, to name a few options. Also be sure to always travel with water. If you do these simple things, then you will not have to turn to the many unhealthy and expensive options along the way. You want to prepare the same way for when you return home.
The second step is for when you get to your destination. Many people act like they have to eat fast food or at restaurants when they get to their destination. It is not like wherever they are going is so far from civilization that there are no grocery stores or other options. Many hotel rooms even have refrigerators and microwaves so there is no excuse why we can't have keep food in our rooms. Make it a priority to find a grocery store and stock up for the majority of the meals for the time you are staying. This step is huge to be sure you stay on track.
Like I said before, it is not like you can't eat out at all when you travel. The problem is that the more you eat out, the harder it is to stick to our proper caloric intake. For the most part, when you are at a restaurant there are not any great options, just better options. These better options are usually around.