What are common ways to say goodbye in Japanese for different situations?
If you want to be poetic, you could say さらば saraba, which is similar to the English farewell - and in its normal sense also used like that, as a last goodbye. E.g. さらば友よ！ saraba tomo yo! - Farewell, my friends!
Among friends you could also use the very informal あばよ abayo, but I would observe your friends first before using something like that. Some of my Japanese friends use it, and then I feel OK using it as well towards them. Its origin lies in the former: さらばよ sarabayo.