An author who's played way too many hours on Stardew Valley.
1. Actually beat the game
I know it's obvious, but have you ever actually played through all three years on one farm without skipping ahead by turning your hard-working farmer into a narcoleptic hermit? Of course, Stardew Valley never really ends, but for the purpose of this list we'll define 'beating the game' as getting married, completing the community center/joja mart checklist, and getting all the candles lit by your dead grandpa.
2. Marry someone else
Or better yet, marry everyone. Now that divorce is a part of the game why not change things up, see all the room additions all the marriage candidates have, their new dialogue, what they give you as presents. This isn't real life, so it doesn't matter if you just drop the father of your kids like a wet sack, after all, you can always turn the kids into doves. They won't even notice, I swear.
3. Make everyone hate you
You've spent years trying to befriend everyone, giving them nice presents, remembering their birthdays, having long conversations with them saying the same three things every single day. But you know what they say, you either die a hero or live long enough to see yourself as that crotchety old man who yells at kids as they walk by his house. Channel your inner George and start being mean to everyone. Give them horrible presents. I recommend algae, after all what else are you going to do with all that gunk you keep fishing up?
4. Cook all the food
I just recently got the achievement for cooking every meal for the first time. It's not an easy one to get but not ridiculous like some of the god-fish or that mini-game. Though it might turn your nice little farmer into an absolute hoarder. Plus you can make some really neat things to help you accomplish some of the other things on this list. Food makes great gifts for people you want to marry, and if you're going the other route there's always Strange Buns!
5. Have a barn full of blue chickens
After you've seen Shane's 8 heart event any chicken you hatch or buy (excluding void eggs and dino eggs) has a chance of being blue. But they don't produce blue eggs, just the normal white and brown. So what you'd do is incubate as many eggs as you can, and sell all the ones that aren't blue, because racism is okay when they're chickens. See how many days it takes for you to have a coop full of blue chickens!
Stardew Valley for the Switch
6. Achievement hunting
This one is a 'do at your own risk' kind of recommendation. Some of the achievements are ridiculous. Looking again at that stupid mini-game I've only bothered to play once.
Ones like cooking all the food, shipping all items, getting 10 MILLION gold, are a bit more doable and even fun even if they take a while.
7. Slime breeding
This is the feature I think is most forgotten of Stardew Valley. I've never even tried it, and I don't see it talked about much. It doesn't seem to accomplish anything in game, but checking it out and playing with it a bit might kill some boredom for a while. And besides, aren't those little slimes just the most adorable little blobs!
8. Make all your money off the cheapest crops
Started a new game and don't want to do the same thing you've done with every single other farm? Challenge yourself to grow and harvest only the cheapest, least valuable crops and sell your soul to Joja Mart. Let's see you get 2 million dollar teleportation wand like that!
Crops would be:
Summer: Summer Spangle
9. That stupid mini-game
If you're truly desperate for something to do you can always try Journey of the Prairie King for a while. Losing on that over and over might just remind you how much fun the real game actually is and kill your boredom stone cold dead.
10. Mod the game.
If all else fails and your still bored check out all the mods for the game. Some make the game more streamlined, others add more content, and still, more are just silly fun things to enjoy for a while.
From turning your horse into a pokemon to outright cheating you can do nearly anything with mods.