Get candy, lots of candy. We got an embarassingly large amount last time between just the 2 of us, in fact, we brought most of it all home and that ended up being my holiday gift for the people I work with!
Oh, now that's an idea - last year we just left it for Mouse Keeping

I would take your own candy bag, *** you can bring a bigger one and get more stuff!

We don't tend to like a lot of the stuff they put in the bags (we're not big sweet eaters), so regularly go to stops to increase our chances of goodies we like!
The Mad Hatters trail, which we only found this last year, is definitely one to go on. Candy, candy everywhere, and a few surprises along the way!
Photo wise, make sure you visit the PP people down by the steamboat, as well as over by Haunted Mansion. There is also Cinderella's carriage which is kitted out with pumpkins and stuff - gorgeous and worth a photo.
Do the second parade if you can, because it's usually much quieter and you can get a better viewing point. Although, generally speaking both parades are quiet compared to regular nights!
Don't miss the Villians Mix and Mingle in front of Cinderella's castle. We always say we'll do it, but some how manage to mis-time it and get just one or two villians.
Hallow-Wishes, well what can I say about that? Put it like this, if a MNSSHP ticket got you into the park just to see the fireworks.. I'd be very happy! By far my most favourite display of them all, and the one I look forward to every year.
We normally do a couple of rides, but not so many, as we like to soak up the atmosphere and figure out who came dressed as who. Speaking of which, it is the one and only, final thing you absolutely must do - dress up! It doesn't have to be clever or difficult or perfect, but if you're in costume you'll have a lot more fun and more interaction with the CMs.
First year we went, I was Tinkerbell. Second I was Giselle, and last year I was Sleeping Beauty. I am still debating over what to go as this year, and need to get a wiggle on about that!
But go, have a great time, enjoy yourself and get into the spirit... see what I did there?!... of the night.