Challenges that I've enjoyed include The Decades Challenge - start in the 1890s and let time move forwards. Does require some CC if you want it to be realistic, especially with so few packs, though Cottage Living offers a good starting point. There are a set of "rules" but I just played along and let history happen to my sims because that's more fun than rules.
A challenge I am working on now is a pack challenge, playing one pack at the time and really exploring the pack to its fullest. Then again, you have very few packs so I'm thinking that wouldn't be as interesting for you as it is for me who has packs I haven't truly explored before.
A challenge I've thought of doing is some kind of survivalist challenge, where your sim is isolated and must survive on what they can craft/farm on their own. There are a few different takes on this one, google and find one that fits you.