How do you keep your game interesting?

I have a few households that I play in random rotation. They are all different and it depends on my mood which one of them I choose to play. Sometimes I build new fun community lots for my sims to use, and I update them quite regularly, either because I get new ideas or we get a new pack with stuff I can use. 

It's an interesting question 🥰
I like to play with several families in the same game. The rotations help keep my curiosity piqued. I play with a mixture of sims I've made myself and sims they meet in their world. I don't care much for their lore. If their appearance or demeanor inspires me, they are in.

I would say that I pretty much keep my game free of any kind of story progression as it destroys my imagination. I'm constantly creating stories in my inner universe as well as by writing Simlit. I love the randomness that occurs in the game that makes the story move in a different direction. There is so much that can inspire me and I never seem to run out of material.
For me, all the big emotions like wants and fear seem disturbing, and the same I would say about story progression. Suddenly I find my Sim in mourning over some random townie's death. A townie I had no idea existed. I take no pleasure in the external drama.
I know others feel the exact opposite and that's fine as long as it brings them joy. Ultimately, this is what makes the joy of the game last.

Definitely rotational play. I have 16 households and play 1 per sim week. I try to have each household doing different careers, different skills, having different ages etc so that I can experience all the game has to offer.

I'm a rotational player, not just when it comes to multiple households but also I alternate between Live Mode and CAS and Build Mode - whatever mood I'm in. I might build for weeks, and then want to play a generation of one of my households. I keep notes of where I leave off so if I am bored and looking for ideas I have a list of what I'm in the middle of. I feel like builds and renovations are never finished! 

Also, a rags to riches challenge always does the trick for me! I get bored once they have money for non-essentials, or a house. I'll sometimes build a tiny house but more often than not just stop and release them to Other Households.  

I’ve been adapting each generation of my legacy so that the next gen almost always explores a different pack. It keeps me motivated to continue which I’m really pleased about bc I’ve never managed to play a legacy before! It keeps things fresh and I love watching my family’s story adapt with the game itself. 

I play on rotation, I switch couple's and have babies outside of marriages. I try to pretend like my sims don't be knowing who their real parent is, like a soap opera plot but of course sims can't keep that kind of paternity secret. 

Mostly mods and making up my own rules and mechanics.

For me playing the decades challenge (my own style) has made the game very interesting. I keep having to find ways to incorporate history into the game and work around it which makes the game more interesting. For instance one character suddenly had a crush on her best friend, and this in a time when homosexuality wasn't legal, so then I had to figure out how to work with that element in game and think about the possible consequences. Then you have to work in all the major events into the game, so how does WWI or II affect the characters? Who goes, who stays, who lives and who dies? For me it's enough to keep me interested, especially as I play the game semi rotationally, so that I can get as many aspects of history in there.