My gaming group stopped meeting for campaigns a while back; we all live in different parts of the country now, so meeting up for regular sessions isn't practical. However, when we are all in the same place, we run one or two daylong sessions. Each of us has written a bit of fluff, and we try to play through one dungeon or storyline that somone's prepared in one day. Often, each DM likes to keep all their daylongs set in one world, to give the feel of things developing with each session, giving it a sort of campaign-esque feel. Generally speaking, the system used is D&D 3.5.
Basically, I'm looking for advice on making a one day session run smoothly, keeping it fast paced, making sure it doesn't feel too railroaded, that sort of thing. The difference between this and a normal campaign session is that I have all the time in the world to put into it, and I'm sort of expected to put in a fair bit. If you had tons of time to put into planning just one session, how do you think you'd structure it? Any related advice is appreciated.
Also, as an aside for any 3.5 specialists, any advice on making level-appropriate challenges? Level 6 Gestalt, three or four players, full casting progressions are banned so level 2 spells are the highest I'll have to deal with. Any tips that help with finding that optimal level of fun where challenging meets doable are much appreciated.
If it's of any relevance, the basic plot is that they are sent on a quest by a powerful spirit (a sort of mini-god, if you will) to gather information on another such spirit after he started getting some rather worrying magical vibes. They'll journey to a massive underground city to do some investigating. There is a fair bit more preamble to it than that, but I shan't bore you all.