That depends on the context.

"What time do you choose?"

"Tomorrow evening, and the evening after."

In this case "I choose..." is implied and both evenings are in the accusative case.

I was wondering that myself. Hopefully someone who knows will answer.

The first one too :) As far as i know in essence that's always the case when the purpose of the expression is to locate the rest of the sentence in time (even when the rest is eluded as here). I guess it could take a nominative case if we wrote, for instance, 'morgen abend und der nächste Abend waren schön, in which case they become a subject'.

Can only be den. That's what i tried though and i lost a heart

"Nach" means "after." "Danach" means "after it." Danach can be used by itself; but "nach" needs another word (otherwise the listener will wonder "after what?"). So in this case, "danach" should be used.