I had each player roll up two 3rd level characters using 3d6 in order. What resulted was great fun. One player rolled up two dwarves - brothers - named Ping and Pong. The other player rolled up Blorg the Fighter with an Intelligence of 4 and Seth the Cleric with a Wisdom of 7.
While they were rolling up their characters I was quickly trying to find a suitable adventure for the evening. I grabbed a handful of Judges Guild modules and started flipping through them for some inspiration.
After the characters were created, I had each player roll a d20 a number of times to determine if the characters had any magic item. I usually have them try to roll under their level (3 in this case) for each of the magic item categories. They actually rolled a number of 2's and had a handful of magic items.
Alas, the adventure was short lived. After coming upon an old keep populated with goblins and ogres, the party spent a number of days travelling with few rolls on the Ravaged Ruins table to give some flavour and a few wandering monster rolls which came up negative.
Finally, I rolled up a wandering monster which led me to the Dragon subtable. Thinking to myself, "Oh, oh...", I quickly rolled up an encounter with two fire salamanders (AC 2, HD 8, 3 Attacks and fire damage to anyone nearby) which surprised the party. Seeing as this was a oneshot and I wanted to see what the players would do, I decided to just spring the encounter on them and see what happened. A roll on the monster reaction table gave the party an opportunity to try to evade. Unfortunately, the PCs decided to charge the fiery lizards and soon thereafter we had a dead party.

What Worked Well:
1. Rolling for magic items gave a great hook for the adventure with the treasure map.
2. Rolling to figure out where in the Wilderlands the adventure would be located was quick and fun.
3. Judges Guild's Book of Treasure Maps was a great help.
4. Rolling up four 3rd level characters with B/X was fast and led to some fun characters (not a surprise here!).
What Didn't Work as Well:
1. I discovered I am rusty at running a hex crawl. Being unprepared for an impromptu B/X one shot definitely contributed to this as did the fact that we randomly determined that the party started in hex 0914 of map one of the Wilderlands. However, it has been a while since I have run a hex crawl. I would like to play in someone else's game for a hex crawl just to see how others do it.
2. It being a oneshot definitely impacted the players' decisions. Four 3rd level adventures with no offensive magic going up against two fire salamanders was a definite mistake, but it didn't really matter because it was a oneshot.
All-in-all a good time and a great way to spend the evening.