I usually always always always play families. But always playing the same thing over and over does get boring. I have this in every Sims game for that matter, there's just only so many times families can be interesting to play cradle to grave (in a row).
So, here's a few things I'm currently playing:
a) Military base: I have 6 Sims (5 now actually but they're looking to recruit), 5 in the military career and one in the scientist career. The goal is to have them each max out their careers but they always need to work from home (except the scientist of course, I follow him to work about once a sim week). They need to complete their daily tasks, and the highest one in rank orders them around. They also go to festivals together sometimes, celebrate important annual events (like Christmas), and also together solved the Strangerville mystery.
b) Asylum challenge: this is a new thing I was trying out. You can look up the rules easily on the forums. However, I'm not 100% sold on it yet because I think I'm too much of a control freak. Having only one sim to play with may not be my thing, but you can try. There's really good lots for it on the gallery.
c) Orphanage: I downloaded Deligracy's orphanage and her sims. I love the lot so much, deleted one of the kids, aged up the vampire. But the rest is the same. The idea is that all kids have some supernatural powers. There's a ghost, an alien, a vampire, one sim that never sleeps, one that is a savant and another very creative kid. The headmistress adopted them all as care dependants. I love playing this SO much! Technically the goal is to complete their aspirations before they age up (to either teenagers or young adults). I do that but I'm not very strict in game. I just love having all these young sims around learning to deal with their powers. I've never even really played aliens or ghosts before, so it's quite new haha. I'm even thinking of setting up my own magical school when we have witches! There's also mods so they don't have to go to school if you're interested.
Challenges can be interesting too. But I think you mostly need to find ways to stop always playing families. Play only adults, or only teens, maybe only supernaturals, just try something you usually don't do! That's how you keep the game fresh. I still have two regular families in this save should I feel like playing them - and for about 2 weeks I've visited them maybe twice ?