Though we naturally think of exercises as a way to stay fit, we need to realize that the key to staying fit while travelling is a combination of the workout factor as well as the dietary factor; the latter of the two is perhaps more important, since travelling necessarily involves long hours of walking and physical activity, though still falling short of being a proper workout.
Hydration is a key:
Most physical discomfort that you come across while traveling can be narrowed down to the body feeling dehydrated. This often occurs because the body tends to lose a greater amount of water at this time as compared to when you follow your normal daily routine, which in most likely hood is sedentary or at least partially sedentary. The key then is to remember that you need to drink that extra bit of water to compensate for what's being inevitably lost. This will help your body maintain a good balance between electrolytes, thus ensuring that you don't witness a drop in your energy levels. Furthermore, good hydration will also ensure that your muscle tissues are rendered elastic and nourished, since water helps in transportation of essential nutrients, as a result of which you will be less susceptible to cramps and fatigue.
Word of advice: Carry a bottle of mineral water with you at all times. Also remember that aerated drinks are not a substitute for water, and in fact contain dehydrating agents and fattening sugars.
On the diet front:
Keeping an eye on the diet becomes doubly important since traveling increases your reliance on fast food and packed food, both of which not only induce weight gain owing to their high fat content, but also fail to provide most vitamins, minerals and essential micronutrients that healthy, home-made food can supply. Furthermore, special care needs to be taken by those who have a sensitive stomach, since even the most innocuous sounding food can have herbs or spices that can act as stomach irritants and play havoc with your digestive system.
While you may often find yourself out of options with regards to what you eat while traveling, you should try opting for restaurants that serve whole meals over fast food joints as often as possible. Besides, you should also treat yourself to a generous helping of fruits and vegetable salads often so that you get the required vitamins and minerals. Opting for baked and steamed foods instead of oily ones is another great idea to avoid weight gain; especially for people with sensitive stomachs, who would be advised to look into the ingredients of a dish thoroughly before they order their meal.
Working out while traveling:
Though it is essential that you hit the gym as frequently as possible to ensure that you don't lose touch with your regimen, it's advisable that you tone down the difficulty level of your exercises. I.e. follow the routine that you had before the last upgrade. This way, you can save up some energy for the rest of the day and also ensure that you don't come down with a muscle pull or twisted ankle.
Another good option is walking as often as possible, since you will realize that not only is it a great way to cut down the extra calories that you may be putting up, but also offers a great way to truly see and experience a new place.