Which Sims expansion is your favorite?
Lately I've been loving Seasons! It gives you a calendar which makes it easier to keep track of tasks
The holiday feature in that one sounds interesting. Can you create your own holidays? 
I think you can! You can create your own events so I bet you could make your own holdays too
Have you played the Cats & Dogs expansion? I'm curious about the veterinarian career. 
Yes! I love it since you can interact with NPC cats & dogs just like real life! lol The vet career is great & they added a major at university for it & skills classes you can take. You can take your pet into the vet & use the machines even at a low skill level.
I wonder how many pets you can adopt in to a single family. 
i think the limit for household members is 8 so one sim could have 7 animals! It'd be a little hard to manage but certainly fun lol
The Parenthood expansion seems like it might be fun as well. If you do well with pets, sim children should be easy! 
haha right?? Parenthood lets you teach your kids more & mold who they will become. It's kind of involved but that does make it more realistic. For most of my games, I keep it to 1 kid, 2 adults, & 2-3 pets max.