A bunch of craziness!!!!
On a serious note, there's panels, concerts, events, game room, and more to go to. Of course there's also all the people in cosplay to check out too
As for pre-registering for a con, it depends on the con. If you register early you can save quite a bit. For bigger cons I would recommend pre-registering to avoid the long lines which can get REALLY bad. Hotel room depends on you. If it's in the same city I would say to save the money on a room, but some people like to stay at the hotel for fun anyways. Guess it depends on if you have money to spare
Just taking cool pictures of people and browsing can take up a lot of time at big cons though!