I don't do challenges, because .... well I don't like rules imposed on me.
But I do use them for inspiration.
Such as the CAS Roller, where your roll 3x and each roll you must include that for your current sim, such as the hair color, accessories, walk style, trait, etc.
I also like the "Do It As You Go" Challenge. It has a nice big list of events to roll every 3 days. I use that for my personal stories, but if my sim is in the middle of something, such as enjoying her career, and I rolled for her to swap jobs, NOPE, not happening at this point in time, but may next time if I rolled that event I would change, if I roll something I don't want, I just re-roll.
I also created a weekly events roller for my personal that includes most of the challenges events (but some I removed because I don't play the game that way) and I also included activities for the various venues that I've placed in my game, such as the drive-in movie theater and the dry-cleaners/laundromat.
I also created a daily event/goal that I roll every morning at the same time and my goal is to try and acheive that within the next 24 hours until I roll a new daily event.
My other self-imposed rule is that I have no skill building items on my lot, this forces me to get my sims out in the world to learn those new hobbies/skills plus they meet new people that way.
I'm also incorporating some other aspects of various challenges into my normal gameplay. I like the challenge of going to different households and being the "hired help" for them for a few days and within a certain time frame you need to "meet" your next family you will move in with to help out.
I also started a self-imposed rule that when I'm playing a household of more than 1 sim, I swap controlling the sims every day and let them just do their thing while I watch (which is a little hard because I want to step in and guide them to do somethng different) but I've managed it so.
Sometimes it's just fun for me to read through challenges and pick and choose what I like about each of them.