Here's an idea for a module.
Scouts, roadwardens, travellers, or what have you report horrible shoggothy monsters attacking an isolated coaching inn, tearing it apart and slaughtering everybody within. They report a team of mighty heroes (bedecked in magic items and other loot) fighting the shoggothy monsters, and being defeated.
The PCs might go to investigate the ruins, perhaps to loot the fallen heroes.
Only there aren't ruins. The inn is intact, the people have no idea what you're talking about, what monsters? Everything is fine.
Except they aren't. Maybe the PCs have been here before, and the interior is different, the staff are gone and replaced with totally different people. Things seem off.
And then when you turn to leave, the door isn't there anymore.
So what's happened?
It's mimics. The staff are dopplegangers, the furniture is mimics, the windows have see-through mimics instead of glass. A flame-mimic burns cheerfully in the fireplace.
Once the PCs wise up it's a pretty simple dungeon to explore, full of lurking monsters in disguise. The frustrated cry of 'god dammit, not the stairs too!' should be heard at least once.
The loot you found? Small mimics. The 'heroes'? Those were dopplegangers, their fall was bait to encourage more heroes so the shapeshifters can steal their appearances too. Escape the inn, and guess what, your horses have been replaced. And those trees? Mimics.
On the other hand, the actual blob-monster they find is a stuffed trophy killed by the inn's founder in his adventuring days, and totally harmless.
Do not, under any circumstances, let the party split, for obvious reasons.
This should play out like a comedy of errors, a bizarre farce where nothing is as it seems. It should be totally ridiculous.