Okay, we played last night, and I took some of your advice with me.
For the tl;dr crowd:
I prepped nothing for the game other than a general idea ("escape" from those chasing them and a sudden violent storm into a haunted crypt) and let the game run it's course.
It went great, actually!
For the chewing on posts crowd:
The party, at the end of the last game, had escaped from a botched attempt at kidnapping an orcish captain of an army, which then attacked and sacked/pillaged a major city and it's army.
On the run from the orcish army, and with the orc captain bound and prisoner, they run in an attempt to warn the major military power in the region of the surprise orc infiltration. They are ambushed by a group of outriders, one of whom manages to escape them (thereby alerting the enemy army...) and are forced out into the rocky hinterlands. Trying to get back to the main roadway, they are caught out in the open by a violent fall storm (the first cold rain of the season) and seek shelter in a "huge earthen mound with what appears to be a partially collapsed stone entrance."
Yup, a barrow, and they thought it was a great idea to make camp inside.
Needless to say, (and this is where I got happy) the rogue of the group goes exploring, gets into trouble, pulls the rest of the group (including a still bound orc warlord!) deeper into the barrow, because of "barrow...barrow...GRAVE GOODS!" I have to hand it to the rogue player (he's the newbie of the group); he really latched onto his character last night.
Anyway, various creepy undead things are awakened by the bumbling around, and some hilarity ensues (throw the only lantern you have at an enemy, inside a pitch-black barrow...okay...oohhh...the enemy wasn't flammable, the lantern oil dies out in a few turns, and none of you have darkvision...but the monsters do...) and there are a few tense moments when PCs got slammed down to 0 hp.
But, all ended well, they defeated a rough group of enemies (really, a brutal scoundrel rogue can dish out damage like no one's business) and everyone had a great time.
After, I asked about some character desires, took some notes, and will run the next game just like that. It was really freeing to just let the game play out as it happened, as opposed to trying to map out every possibility.
Also, it seemed that as the night wore on, the players became more comfortable with the idea of "no script, just do what you would" style play. They aren't totally comfortable with it yet, but I hope they will be soon!