Traveling with Instruments
For any musicians who decide to travel with their instruments, extra care needs to be taken when you decide to go international. Within the United States, the current rules state that you can bring a violin or guitar sized instrument on board, and provided there is space in the overhead compartments (board as early as possible!), and you will not be required to purchase a second seat or asked to check it. However, on international flights, the rules can vary more widely by airline.
Generally speaking, if you have an instrument in a case which is small enough to be considered hand luggage or a "personal item," there are no extra steps to take. While instruments like violins usually fall into this category, even with a larger hard case, most guitars won't fit into this category, nor will other, larger instruments (such as larger stringed instruments like cellos). If you aren't checking your oversized instrument (which is really not recommended, and often not allowed by airlines), you should call your airline ahead, as you will have to purchase a seat for your instrument in the cabin itself.
If you're looking to take something like a guitar abroad with you, it may be worthwhile to look into a travel guitar, which is what I brought with me when I first came to Germany. They're available either in acoustic or electric (I'd go with the electric as they're smaller, and less prone to damage in travel, particularly if the purpose is primarily practicing). Depending on the model that you pick, some of them can be placed into your checked baggage without increasing the weight or taking up too much space.
Realistically though, if your primary goal is to stay in practice while you are away from home, and you don't already have an instrument designed to travel, it would also be worthwhile to look into practice aids for your particular instrument, such as practice fret/fingerboards, which are available for many stringed instruments. You'll be happy for the extra space in your baggage when you return home, as you'll find that you accumulate more things to bring home than you might think over the course of your time abroad.