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 an event	passé composé	Le temps a passé si vite. — Time went by so fast.
back, at how things were	imparfait	Quand je suis arrivée, je ne parlais pas bien français. — When I arrived, I didn't speak French well.
back, summing up	c'était	C'était une année inoubliable. — It was an unforgettable year.
at today	present	Aujourd'hui, je parle français !
ahead, soon	aller + infinitive	Tu vas nous manquer. — We're going to miss you.
ahead, a promise	futur simple	Je reviendrai. — I'll come back.

To sum up an experience, French very often uses c'était: C'était génial ! — It was great!

Words for the occasion: inoubliable — unforgettable; remercier — to thank (Je veux vous remercier pour tout.); promettre — to promise (Je promets de revenir.); pleurer — to cry; revenir — to come back, future je reviendrai; ne … jamais — never (Je ne t'oublierai jamais.).