Geese! They’re in a small market at the edge of the wilderness where people get supplies before heading in.
Suddenly, a small flock of geese (use whatever medium sized flying monsters of appropriate CR) led by a big black goose swoop in and start stealing stuff from market goers! It’s a chase-fight!
Have each goose explode when “killed”, and obscure a 5ft radius area for a round. On their turn after the “be a gas full of feathers” turn, they re-emerge as quicklings, laughing and zipping around the outskirts of the fight.
When I did this, the players didn’t mess with the quicklings and focused on getting the black swan, and were each rewarded with a gift (a healing potion in the form of an apple, and the daylight spell held in an acorn) and the group got a clue in the form of a cryptic poem that helped them navigate the labyrinthine tunnels under the city later.
This was the big set piece of the 1-hour adventure I ran at a library mini-comic-con a few years ago.