How do you keep it interesting?

I focus a lot on community aspects, such as laws, inheritance, traditions etc, so there is always something I can look at or look forward to. With seasons my year span got logical and it helps a lot. I have a functioning detailed economy system, and I have a regional council with elections every 5th year. I play on rotation, so I can always switch to whatever house I like, meaning I never get bored with the same house for too long. It also means there are lots of features to test when I get a new pack and say 1/4 of the households should be affected by either game features or simply an upgrade of furniture/CAS stuff. If I need more time to check out new stuff, like sending sims off to explore the Selvadoradian jungle, I'd simply play one extra calendar year without aging the community. I pay a lot of attention building each character, with back story, interests and perhaps a conflicting personality. Make sure to give them goals, what do they want from life? What makes them more happy?