Adventure Idea: The dwarven cleric in the group has a rather unique mount. Instead of a heavy war horse like the rest of the group, he's selected a war trained bison. Not a bad choice from a stat perspective, since it's a better CR monster and costs one tenth the amount of a horse. Thematically I make sure to describe how bad it smells and how many flies it attracts to keep the balance compared to the rest of his group, but he's pretty fond of his war cow.
So as a random encounter, I plan to have the group meet a lone traveller in the morning as he walks into their camp at daybreak, a few hours before the group are ready to start travelling. The old traveller asks to share their fire and brew a pot of tea. Assuming they accept the offer, he'll make small talk, then eventually express interest in the cleric's mount. He'll say he's been looking for a reliable steed and the war bison would suit him perfectly, assuming the cleric is willing to trade for it.
So, what's the old traveller offer our cleric in payment?
A handful of magic beans.
Of course, assuming we reach this far without our players getting paranoid and avoiding the encounter entirely, the players will definitely pick up on the obvious Jack and the Beanstalk reference. If they take the deal, the traveller will depart with his new mount, and instruct them to plant the beans at sunset. The next time they strike camp, they'll discover the war cow wandering into their camp, along with a golden holy symbol hanging off the saddle.
Functionally, the beans act as a single use magic item of permanent Teleportation Circle when planted, save that it requires an easy Climb check to ascend up the beanstalk into a permanent cloud at the top, leading of course to a 'dungeon' full of giant typed creatures.