bye-bye

大事に(おだいじに)	If your friend is a little sick (but not, say, cancer), instead of “byebye!” you say お大事に (take care of yourself!)

さよなら	Sayonara - kind of formal and only used if you’re not going to see the person for a long time.

気をつけて(きをつけて)	Say this if your friend is going on a trip.

いってらっしゃい	Say this if you are living in the same house as someone and they are leaving the house.

またね	The casual form of “bye-bye!”