If you have two sims enter certain rabbit holes together, they can woohoo in the rabbit hole! There is a chance they will get caught and be kicked out, still naked. They will get an embarrassed moodlet from it.
Depending on their traits, your mail deliverer might angrily throw down your bills instead of doing the normal animation.
Sims can become friends with their cars. They can even go so far as to becoming best friends forever. Sorry for low quality image, im using google skjskjskjs
Once you reach level 3 of the handiness skill, you can program your doorbell to have a different chime than the default.
Reaching a high enough level in the music, or business career, or having a high enough celebrity status will get you a limo escort around town.
In The Sims 3, not only can your fish breed with one another, but they will also eat each other if you are housing a fish with it's preferred bait.
Trying for a baby in the treehouses will have a chance of affecting the traits of the sim that is born. A princess treehouse baby may become a snob, a futuristic treehouse baby might become genius or eccentric for example.
Once you've reached the top of the music career, you may be presented with the opportunity to sign autographs outside the theater.
Sims in the political career will be required to raise money for their campaigns. You can throw fundraisers or just ask random people for donations. You can also embezzle funds. The funds you have raised are kept track of and accurate.
Babies, toddlers and children that sleep with a stuffed bear in their inventory will get the cuddle time moodlet.
Sims can be arrested if caught tagging public places, caught stealing, or by random chance if they are in the criminal career.
I have a LOT more to add to this thread but I’m at work right now so it’ll be a while before I add more

Ok I’m ready to add more. Sims in the journalism career have a special interactions with the cinema once they’ve reached a high enough level. They will be able to review a movie.
If your teen sim becomes BFFs with another teen, then later makes another BFF, the original BFF will accuse you of betrayal in a pop up. I don’t have a pic bc I’m lazy just trust me xoxo
I’m gonna end this thread. I know it was short but I just made it to inspire ppl to play this game, so I don’t wanna spoil any other potential secrets you find! Also check out my YouTube channel if ur iconic https://www.youtube.com/user/72arj
For anyone who wants to find little secrets like this my advice is to not look for the same type of features as the sims 2. You won’t find a ton of detailed animations or subtle interactions btw family. But there are so many features to the careers in this game (1/2)
And the open world. Explore the game and don’t go into it with preconceived ideas of what the experience will be. The sims 3 has been given a reputation of being fun but undetailed. But it’s very detailed in different ways than the other games.
Also fuck it lemme do an expansion giveaway! Use this tweet linked to enter.
https://twitter.com/litrallyaj/status/1295982767260475393?s=21 https://twitter.com/litrallyaj/status/1295982767260475393
https://twitter.com/litrallyaj/status/1295982767260475393?s=21 https://twitter.com/litrallyaj/status/1295982767260475393