The bubble blower. I love it. I love how sims react to it, and how sims react to whether or not a sims succeeds or chokes on blowing the bubbles. The effects it can give to your sims depending on what type flavor you use is endlessly hilarious to me. I love how it takes sims time to get good at it, as well, and how eventually they'll start using it on their own.
Scouting. I almost always make my children sims join the scouts. (I did not like scouting in the real world.)
That water storage science container from Get to Work. The one where in the default Scientist work build it had a gnome inside. My sim's husband was killed by a vampire once, and so I put the remains inside this tube and I ended up with an urn floating inside a science tube. It was immensely stupid and it made me so happy that I could 'store' the dead spouse should his scientist husband come up with a way to revive him one day.
The Parenthood traits that young sims can earn based on their experiences and how they're raised. I love it so much.
I recently sent my sim to a spa and I'd forgotten just how legitimately relaxing the pack is. I never wanted to leave the spa.
This is silly, but I also adore the nudist lot trait. You haven't lived until you have a nudist spa and Don Lothario disrobes specifically before teaching a yoga class. Pure gold.
The aquariums. I could watch the fish swim around for ages.
The Jealous trait. It's so powerful and it's one of my favorites.
