Our theater had...let's see if I can remember the trailers prior to the movie....
Moviefone commercial....Cinderella Story.....Sleep Over....Polar Express....Cat Woman....Around the World in 80 Days.....I think that may have been it.
As for staying through the credits.....no, it's not really necessary like it was in Chamber of Secrets....if anyone wants to know what "happens", just PM me and I'll tell you. It's really not a big deal, and it actually "happens" at the beginning of the credits too, but I don't want anyone to be extra touchy about spoilers.
And yes, I'd get there super early. As I said in my other post, our theater sold out straight shows (and expected more sell outs). I know, I know...."but we already have our tickets!" Right, you do, but you also need seats. And people will, undoubtedly, be lined up and reserve, I would guess, groups of seats, especially people going with lots of kids.