Not sure if this has enough variability to be viable as an incentive, but I keep thinking about it, so I'm suggesting it lol
Weather/Forecasters - especially with how the weather has been recently, this year the UK had the wettest May and frostiest April on record and I know Summer gets crazy in the US, so a weather theme seemed appropriate
Basically the Quirlicorns are born with the innate ability to mimic visual weather effects using a glamour/very minor illusion magic. Not enough to say they have Weather Manipulation or actual Illusion magic, just a little visual quirk for them.
Things like; rain trailing behind them or soggy/wet hoofprints even with no water around. A small sun-like orb that they can summon, can make clouds or haze/mirage effects around them. Snowfall or hailstones, lightning. Sun-rays through clouds, tornadoes, leaves on the wind, frosted glass, steamy breath - if it's a product of weather then it works.
Could be part of the patterns on their coats (like how the food and butterfly incentives can completely cover their other genes) or floating around them, like little summoned illusions.
It would be a permanent choice or at least restricted to the right temperature, so if you choose a sun-orb then you might be able to have the mirage/heat haze effect as an alternative, but that might be tricky to track for chimes, so maybe it should be restricted to once choice, but the effect could be either on or off as the Quirl chooses.