A goodbye is a small summary of everything: what happened, what it was like, where you are now, and what comes next.

Looking…	Tense	Example
back, at a period that's just ending	perfecto	Ha sido un año inolvidable. — It's been an unforgettable year.
back, at a finished moment	indefinido	Llegué hace un año. — I arrived a year ago.
back, at how things were	imperfecto	Cuando llegué, no hablaba español. — When I arrived, I didn't speak Spanish.
at today	present	¡Hoy hablo español! — Today I speak Spanish!
ahead	ir a + infinitive, or the future	Voy a volver. Volveré. — I'm going to come back. I'll be back.

A few words for the occasion:

• inolvidable — unforgettable
• dar las gracias por — to thank for: Quiero daros las gracias por todo. (also agradecer)
• prometer — to promise: Te prometo que voy a volver.
• llorar — to cry
• irse — to leave, to go away: Me voy mañana. — I'm leaving tomorrow.