I'd like to pick your brains, What are some interesting plot ideas that you guys have come up with?

I am a DM for a 5e game in a somewhat generic high magic medieval fantasy setting which is more or less going to be a sandbox kind of game, however, I do want some ideas for plot (with hooks) that the party could pursue. Not looking for world-ending stuff, but monsters to fight, bad guys to defeat and reasons to help the helpless!

I'd love to hear what you guys have come up with for your games, perhaps tell about how your party beat the encounters you set up for them so I can add to it and make the parts that they found less enjoyable, more enjoyable!

So I don't leave you guys empty handed, I'll offer a quest-line that I've ran previously, of my own making.

The party arrive at an Inn along a rural road. With no other buildings around. Inside they get ambushed as the inn was used as the a hideout for a group of cultists. The party defeated the cultists in battle and searched the plays for clues about this cult. In the basement they find more cultists and loot, among the loot is more information about this cult, who they are and their beliefs.

In the next town they arrive in, they learned of some kids who got kidnapped by cultists, After talking to the parents of the kids they try and find the kids in the nearby wilderness, where they were last seen. They learn that the cultists that kidnapped the kids are of the same cult as the one they found at the rural inn. They find one of the kids in the woods, he had escaped the cultists, but the other kid was still taken. They follow the trail that the cultists left behind and find an abandoned mansion. Inside the mansion are many cultists, but the mansion is also haunted, filled with undead. Fighting through the enemies they find the ritual chamber. Where the kidnapped child is being used in a blood ritual. The cultists are concentrating on the ritual spell and there's only a short time limit. If the party is too late the child is turned into a powerful evil weapon. Which the party can choose to keep if they defeat the cultist boss. My party was too late, but decided to keep the weapon in their bag, not using it. But finding a way to turn the weapon back into the child. Eventually, they succeeded at the end of the campaign.