S&W or LotFP old school rules.
It's a low-level adventure, and with everything I put into it, I am constantly thinking "Sleep spell." And then I realize a few things that work against the broken power of old-school-clone Sleep:
* Splitting up groups of low level monsters into multiple encounters
* Having close quarters and enforcing a "friendly fire" rule
* Foes approach from many angles
* Not all foes can be slept
And more ideas in this discussion ranging from a full-on nerfing of Sleep (saving throws, fewer creatures affected) to a "D&D is right" approach. Given it's a one-shot I am not really inclined to accept answers that come down to "it will all wash out over time". And for a longer campaign I would much rather start from a balanced spell system that gave starting spellcasters two or more things to do. In fact, I think the time is right to start discussing magic systems.