I've never really made a challenge before, so I don't know if this will be too easy. I started a file yesterday for this. I just left the mailbox outside. I gave my sim the Childish Trait and a Blarrfy, and so far he's been able to keep up his social pretty well. It tanks pretty hard every time he sleeps, and once money starts becoming an issue it will tank even more. So I'll let you know how it goes.
Did I post in the wrong section? I'm sorry. XD I usually post in General Discussion just because it seems to be the most generic "let's talk about Sims 4" place.