Try to keep it to thing that have been confirmed and not theories. Figured out who Jon Snow's parents are while reading the first book during Ned's dungeon pov. Turned out to be the common consensus. Figured out Artisan Whitebeard = Barristan Selmy. Figured out the queen of thorns was the one who poisoned Joffrey.
i literally figured nothing out until other readers brought it up. that being said, the only one i wasn't surprised by was the queen of thorns poisoning joffery.
Spoiler Lyanna Stark and Rheagar Targareyn. Probably doesn't need a spoiler, but since it hasn't been confirmed I put it in one.
I didn't realize squat. Too many prophecies, stories, etc running through my head. The worst though was the end of ADWD with Mance
I kind of theorized to myself that the Hound was actually at that Monk place before I read any of the theories online.