>>96184529 (OP)Quickly sketch a dungeon and a media-res intro. I feel you, because that's how I feel every game day. Write a short intro of about 150 words explaining where they are, why they're together, and what their objective is "you were summoned by the Quest Giver to rescue the Plothook from the Bad Guy. As you approach the Scary Place a buncha Thematic Stuff can be seen. When you get closer, suddenly Ambushing Enemies errupt from Interesting Terrain and you find yourself Rolling Initiative."
Do a simple ass dungeon and prepare a blurb about what happens when the succeed and get back to the quest giver. Throw in two interesting, weird magic items and voila: your players will occupy themselves for the next 6 hours, no problem.