This is one simple, yet captivating in my book. I personally playstyle, but it has that little challenge twist in it and you can set a goal for it.
1. Create at least two sims (family is optional) in a household and move them in an apartment. One of them must be a romance sim, that will play role in this challenge (let's call him Bob just to make to refer to it, while the spouse name would be sally).
2. Sally must not form any social status with the neighbors, because they are apart and obviously involved in the secret affair.
3. Sally must have a job/career of any kind that allows her to leave, while Bob must be unemployed.
4. Sell any goods your sims earns from good dates (because that will arise suspicion of the spouse and you don't want that & thus don't keep valuable items).
(5. I put this in brackets, since it's a additional optional rule, which If you want to strict yourself more just to spice up the challenge you're playing: You can only woohoo sims at your own home unit or at community lots only but with the catch of having to return home early before spouse's comes first from home).
If you don't follow and break the 4/5 following rules, you officially lose the challenge.
[Goal: ] Woohoo all neighbors as Bob from the apartment the couple is resided in OR If Bob has the 'WooHoo with 20 Different Sims' LTW, complete that task instead.
~ Happy simming ~
P.S. I do recommend playing this challenge on a apartment that has loads of apartment units, If you have a computer that can handle that many npcs and doesn't run into magor lag/performance issues.. The More Neighbors you have, the more fun/challenging the game gets. :P
I have an idea for an additional rule? If you were to do a point system, you could do + # if you can complete the challenge with no other sims catching you 'cheat' and - # per sim that catches you cheat??
# = amount of points
Great challenge idea btw; I don't play around with romance sims enough