My favourite is the mimic accidentally/on purpose placed inside a bag of holding. Any time the party take something out of it there's a nonzero chance it'll be a mimic instead of what they were after.
Seems like a good way to grant the party a bag of holding early on. They just come across it discarded in a corner somewhere, and are stunned that an item in such good shape was just abandoned. It appears brand new and has a mix of mundane objects and apparent treasures inside, and the words "Bran's Bag of Holding" is stitched in elegant lettering on the outside. If they try to dump it out to do an inventory, one random object remains inside. It takes a very high skill check to notice this, as the mimic mimics the inside of the bag as it holds on. If they dump the bag out a second time, the assortment of objects is the same except for a single item. Asking around reveals that Bran was a famous local adventurer who disappeared suddenly.
Funnily enough, i'm doing a Roll20 campaign with a group of friends, and the setting is that their adventurer's part of an ingame adventurer's guild (helps explain new members and real life absentees).
I was thinking about having them meet another party that's part of the guild since my players aren't the only ones whose a member.