Wait, what's the surprise?
Its the first GW2 holiday so… It’s a surprise! :P
Only Anet knows and they are probably not going to give you more information. I did see in one of the halloween posts about the Mad Memories book backpack not being the only reward for completing an act though. It was in one of the threads about transmuting the book. You should definitely complete the scavenger hunt that starts by the fountain in LA though… end reward is the aforementioned book-pack. Act 2 should start sometime tomorrow.
Free of Fate
Sanctum of Rall
We’re one quarter of the way through it. And it’s the first quarter, so we’re not anywhere near the climax. I can’t imagine that the other parts of the chain won’t have their own rewards.
The finale prize is a facial skin that makes your character look like they have cavities. XD