(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.
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!”