I adore toddlers, first of all. They're cute and funny and just the right amount of fussy (even those without the trait) that they're a challenge, especially if there's two of them. I love how they interact with their siblings and with pets in the house - the imitating especially is so much fun to watch.
Gardening is super fun, especially in the early stages when I'm playing a game and just trying to set up a garden and collecting all the plants.
I've only just recently got Seasons and it's so so very pretty to see snow in winter (and cute to have Sims do their "ooh, first snow" reaction). I love the way the weather makes gameplay exciting because it's unpredictable (at least to me it is). And it poses a fun challenge to gardening.
Also, same as Writing_Reg, I love seeing my Sims do the things that I've had them to a few times, but do them autonomously. I have a child in one of my saves who spends so much time outside on the monkey bars, both of his Dads wander off to tend the garden when they're left to their own devices, his older brother is at the piano a lot. It's great to know that they learn direction and then follow it. (and that kids set tables voluntarily and clean up when their responsibility is high)
Oh and I love pets. They're so very cute and fun to play with.
All that said, my favorite part is still building. I love figuring out layouts, decorating rooms, challenging myself to builds that I wouldn't feel like I want to do, like builds to fit the architecture of a world *cough*Windenburg*cough*