Bell, Book, and Driftglobe is a self-contained adventure that scales cleanly for parties from 1st through 5th level. A treasure map leads you to the ancient ruined city of Ganoksreach. What starts as a search for forgotten treasure in a frozen crypt turns dire as it is revealed that an overconfident necromancer has awakened an ancient evil better left undisturbed!
This adventure runs great as a convention-style one-shot, an introduction to Dungeons & Dragons for new players, or a few hours of quick, low-commitment roleplaying. Just roll up some level 1 characters, skim the document, and get adventuring!
It can also make a fun addition to your current long-running standard campaign or hexcrawl. The site of the ruined crypt should appear somewhere deep in a frozen wilderness, but your players can come across a map to the site in any merchant stall, monster hoard, or as a reward for completing another quest.