1. 수고하세요!
It's like saying "Keep up the good work!"
Technically speaking, you should not use '수고하다' to higher-ups, but no-one will be offended if you use it when exiting a restaurant (식당), cafe (카페), or convenience store (편의점)!
2. 감사합니다.
"Thank you!" - quick and easy!
3. 안녕히 계세요.
"Goodbye" - this is another common phrase you can use!
4. 또 올게요!
"I'll be back!" - If you really like a place so much that you decide to become a regular, let the owners know you'll be back with this phrase!
See an actual scenario with Jin leaving a Korean cafe!