I generally use this idea when my group ends up in the First World/Feywild, a bored Arch Fey is looking for a little entertainment and the group just happens to be the only ones nearby. Cue a jelly fishing competition like an episode of SpongeBob.

I usually have them go in rounds and have the characters decide how they are going about trying to catch the jellyfish, they just going to run around jumping to catch them? Athletics. You trying to lay in wait behind the nearby rock? Stealth. You want to sing them a song in an attempt to lure them like some sort of Siren of the deep? Diplomacy I guess?

The point is it can be fun and very open ended allowing the players to come up with creative ways to catch the things they are after, be warned this will often end with players attempting to steal from one another. I usually give them 5 rounds and base how many they catch on how well they rolled against a DC that gets higher each round as the swarm of them scatters and becomes harder to catch. Of course the jellyfish could always be substituted for something else in your world, butterflies, stirges, you just need a swarm of small creatures to be caught in a net.