I don't know if you plan to hit a lot of the treat trails or stations, but the little bag that you get upon entry to MHP won't be enough to contain all of the candy you will collect if you want to stop at many stations. I would suggest bringing an extra bag or two (a tote bag, backpack, something like that) or buying a tote bag in the parks. There are usually 2 or 3 kinds of Halloween tote bags for purchase in the parks. One of them runs close to $5 or $6. The other type of bag lights up at night with the push of a button, and it generally costs around $10-ish.
If you only want to stop at a few treat stations, in addition to the AP treat trail that theluckyrabbit mentioned above, I would suggest doing the one in the Golden Horseshoe (because there are extra decorations inside) and in Pixie Hollow (because it's very pretty at night). Other people seem to like the Village Haus trail quite a lot.
A lot of people suggest waiting until close to the end of the night to collect candy, as the lines are shorter then and the CMs give out more candy at the end of the night.
The lines for characters are apparently quite long this year (they are usually long, but it sounds like they could be slightly longer this year) so if you have a preference for who you have to meet, i.e., Jack Sparrow, you might want to take care of that early on or be prepared to devote a chunk of time to waiting in line.
If you don't have time to see both Paint the Night and Halloween Screams during the party, I would stick with Halloween Screams and skip PtN.
Remember that there is a pre-party in Toontown as well (which is, apparently, a madhouse on certain nights), and that will start one hour before the "real" party officially begins (so on your night the Toontown Pre-Party should start at 5 p.m.). That's where you will find Mickey in his Zorro outfit, and some colorful "witches," among other characters.
