"Would you like to have something else? Perhaps a piece of cake and a cup of coffee?"
Translation:Haluaisitteko vielä jotain? Ehkä pala kakkua ja kuppi kahvia?
the explanation is in the long message by lk_ here https://forum.duolingo.com/comment/39835666?comment_id=40150374 (and personally I do not understand it because I'm not advanced enough in my study of the language)
Not really. The word else is here in the meaning muuta so you could say haluaisitteko jotain muuta. That is what the waiter would say if they can't give you what you originally ordered. In the situation in this exercise you have obviously reached the time for desert and the waiter is therefore asking haluaisitteko vielä jotain