Dandy is a character that you can talk to for some time trial challenges, similar to the time trial challenges that you can find when exploring, except 1 Dandy can give rewards multiple times. There are only 4 of them and they are all in Liyue region.
Most of her time trials are gliding ones, except for the one in Wangshu Inn, where you have to reach the top of the inn within a few minutes. Each time you complete the challenge, the time requirement gets lower, making it slightly more difficult to complete.
You can keep doing the challenge as many times as you want, but you can only claim the treasure rewards from her up to 3 times. The chest rarity starts at common, then goes up higher each time you complete them.
In Yaoguang Shoal, there is a Dandy NPC that's quite close to the Statue of the Seven - Geo.
The one in Wangshu Inn is at the ground floor, close to the bridge that goes into the inn.
North Qingce Village
This one is on the area that's just north of Qingce Village, near the mountain walls.
Southwest Qingce Village
This area is west of Qingce Village, on a small separate island. Head south from the teleport waypoint and you will be able to see her near the edge.