I'm feeling pretty nostalgic, so forgive the long post....
I was super lucky that my father loved to travel and was an amazing travel companion. My first overseas trip with him @ the age of 10 was a 6-week summer trip to Europe--which started with a week in Paris and continued via train and ship to 4 other countries. What I learned on that trip I have never forgotten...
1) Don't be afraid to speak a foreign language, it's a great ice breaker and you learn something new every day
2) Be polite, gracious and mindful that you are in some else's backyard. Say thank you (a lot) to your hosts, also to your parents, sig other, whoever you are traveling with--be grateful of who you're with & what you're experiencing
3) Try new foods (even if they look weird) but never stuff yourself--nothing is worse than a stomach ache on a trip!
4) Be ready to Walk a lot on cobblestones (so you can eat ice cream every day--and I did :) so you better have comfortable shoes
5) Write in a little note pad ( doesn't matter what, just write what stood out or you did that day)--those old note pads become a precious connection to your youth
6) Know where you're traveling and pack appropriately--just because it's summer it doesn't mean it won't get cold
7) Don't take any day for granted, you never know if you get to go back to the same place with the same person.
8) Take lots of pictures--and make sure you frame some of them ( I still have the first pic I took of my father @ the Eiffel tower next to the last picture I took of him)
What I have learned since as an adult who likes to travel is that
1) You may never have the same leisurely pace or time that you did when you were traveling with your parents--so again enjoy every minute of every day you get to travel someplace new with your family
2) On that same vein, if you can squeeze another day or two into your trip, do it, it may be hard ( work, budget, other obligations), but DO IT, again--you don't know if you will have the chance to go back to the same spot with the same person/people,... you're never going to miss extra days at work, but boy if you only had that extra day in Paris, or Brighton or wherever ...
3) If you're traveling with friends, Make sure you know them before you embark on a journey with them---nothing takes the joy out of trip faster than an ill-matched travel companion---If you like to chill and your friends like to be on the go and cover a lot of territories, you will have problems.
4)Buy a little something to commemorate your trip--they help me anchor a moment that encapsulated something special about that particular trip ( it could be a magnet, it could be a ring, it could be a keyring or anything you like)..
5) Almost forgot, PACK LIGHT!! don't waste your time and your shoulders dragging suitcases from place to place--pack light and leave room for souvenirs
6) Stay hydrated, water is more important than food when you are traveling
7) Have a realistic budget for your trip (especially for those long haul ones)
My singular motto to my younger self would be---BE grateful that you can travel, make sure you do so with people you adore, travel as much as you can comfortably afford and add one more day.