I'm not sure why, but this idea for a short vignette as part of a sandbox campaign popped into my head last night:
The Hillfern Wedding
As the players approach the town (Hillfern, population 750, swollen to over 1,000), they realise that something is going on - they can hear the distant noise of laughter, music, and merrymaking. Finally they arrive and discover that most of the population of the town and the surrounding countryside is celebrating a wedding. (If they ask around, they'll find out that it is between the son of the mayor and the daughter of a prominent merchant, whose fathers have put on a feast for the entire town in a show of magnanimity.) It's mid-afternoon, so the population is in an advanced stage of drunkenness, as they've been reveling since morning. There are stalls everywhere hawking wares, ribbons and pennants festooning the houses, young couples getting up to who-knows-what in the alleyways, and vomit in the gutters.
Roll a d20 on the following table every so often while the players are in the town:
1-2: The players come across two men having a fight that has gone too far, as one of the combatants is just about to draw a knife...
3-5: One or more of the players spot a pickpocket stealing from a middle-aged woman's purse.
6-8: Passing a narrow alleyway, one or more of the players notice that somebody has dropped a pouch a few yards into the entrance, out of which a few gold coins have spilled. This is a trap: Two cutpurses (1+1 HD, Club) are waiting in the shadows to ambush anybody who tries to pick up the gold.
9-12: A pickpocket targets a random party member.
13-16: Close by, a horse takes fright, suddenly cantering off down the street while the panicked young noblewoman who was riding it clings on for dear life.
17-19: Somebody tries to assassinate the father of either the bride or the groom.
20: The party run across an old enemy.
Of course, this has limited mileage. Probably you could only make one roll on the chart as it stands before the players start to think, "Just what kind of a wedding is this?" - and a roll of 17+ will obviously lead to something bigger than just a vignette. The chart could do with expanding, too; I jotted it down roughly in the space of five minutes. But you get the idea, I think.