Jeff Rients has a most effective article on coming up with an instant dungeon in Fight On! magazine issue number 6 most appropriately entitled HOLY CRAP! I NEED A DUNGEON RIGHT NOW. I find myself so much in such a situation the day before I committed to run a dungeon and just emerged from a very toxic work week. I do heartily recommend this article if you find yourself searching for some solid tips on how to deal with these last minute crunches.
One such piece of advice deals with quickly but effectively stocking your shake and bake dungeon with monsters which are hopefully going to offer something for your players' characters to sink their fangs into. I am a fan of customizing my big baddies before I throw them at the players as I always believe in tossing them something they've never encountered before as opposed to just recycling something straight from the monster manual.
Jeff suggests rolling once on the appropriate encounter table for your dungeon's level for the basic monster and then rolling again on the encounter table. After this, simply amalgamate the two monsters who have rolled up.
I decided to take this for a spin during my last Traveller-themed dungeon crawl. The results were pleasantly intriguing (and surprisingly fun too for my players). First roll on the wandering monster table under Labyrinth Lord rules produced the Owlbear- my usual favorite. Second roll was the Gorgon: mean and deadly considering its petrification attack. Amalgamating the two together produced a monstrous and deadly hybrid- the Owl Gorgon. It looked, smelled, attacked and died like an Owlbear but packed the hideous surprise of breathing out a cloud of greenish gas when it had you in a bear hug which, if you failed your saving throw, turned you to stone.
As it is, my players wisely ganged up on it and applied massive dollops of shock action (ie. cut it to pieces before it really got to turn someone to stone). The fighter narrowly missed his save but never really found out what was going to happen to him if he failed. Too bad he later got careless and failed his save against a corrosive poison gas trap later on as he fixated on looting a magical sword. But that's an altogether different story.
As to generating the Owl Gorgon, I got it down pat in 2 minutes flat. Not bad for a harassed dungeon master.
Jeff's advise shows you that thinking slightly out of a box and using the tools provided in existing old school clone rulesets will really yield great dividends. A great savings in both time and effort.