Encourage adventure and fun with this goodbye letter for travelers.
Dear {Recipient},
Henry David Thoreau once said, "Go confidently in the direction of your dreams. Live the life you have imagined." I am so proud of you for taking this major step in your future, even if it means going all the way to {place}. You're going to learn so much and have so much fun. You could even bring me back a {cultural item} if you felt so inclined.
Now, don't mistake my approval for enthusiasm. I am so sad that you are leaving for {amount of time}. I think that is too much time where I will be denied the pleasure of seeing you. I will always be here, though, if you ever get the chance to write a letter or make an extremely expensive phone call.
If you ever feel lonely or out of place in {place}, just remember that you are on an amazing adventure. Not many people have the resources or courage to do what you're doing, and you'll return a more learned and successful person for it. When in doubt, just remember the immortal words of Douglas Adams:
"I may not have gone where I intended to go, but I think I have ended up where I intended to be."
Sincerely,
{Sender}