I've got over 200 hours in just regular, fresh out of the box Stardew Valley. I could probably keep on for another 200 without ever modding the game, but I've had my eye out lately for mods that would spruce up my I-don't-remember-how-manyth farm. Be My Valentine by Taskmaster on Nexus Mods is a small one, but just in time for actual Valentine's Day which is pretty sweet.
On Valentine's morning, your farmer will receive letters in their mailbox from friends and love interests each with their own message. Maybe even a gift. They seem to have the silly puns and themed jokes that I remember from handing out actual boxed Valentine cards as a child.
A big factor in my mod choices lately is how quickly I'll actually see what's been changed in the game. Mods that add endgame content are neat, but there's a huge risk that I'll get distracted and wind up ditching my new playthrough before I see any of it. Be My Valentine is perfectly timed because it takes place on the 14th of Spring. You'll get your first influx of pink-colored cards just two weeks into a new farm.
Taskmaster doesn't specify how many hearts you'll need with an NPC before receiving a Valentine. Since the event is only halfway through Stardew's first season, prior even to the Flower Dance festival during which your farmer can ask a potential date to dance, I wonder if I'll receive any cards at all in my first year. Perhaps the requirement is low, though. They are just silly Valentine letters after all, not a Mermaid's Pendant for proposing marriage.
Taskmaster has a few other fun mail mods for Stardew that you can try out. One sends silly Junk Mail letters to your mailbox like ads from Stardew's local businesses. The other is Monthly Mail which delivers lore-friendly periodicals like the "Fruit of the Month Club" or "Mining 4 Ores." They're all small, goofy additions but often what I need out of a game I've played for hundreds of hours is a new chuckle.