take some time to process your feelings before you start writing so that you can remain as polite and professional as possible. In a professional setting, use a formal tone to wish your coworker success in his future endeavors. If the letter is intended as a personal good-bye...
You purchase a gift package, which is basically a gift card for the experience of their choice. Prices range from (give or take) under a hundred bucks up to about $300. The recipient can redeem it for whatever adventure they choose, which could be a hot air balloon ride the next town ...