Which stuff pack adds the most gameplay?

I'd say Toddler Stuff. Not sure if you play with toddlers and families, but it has multiple new playground items for toddlers. Slides, the much-maligned ball pit, and the tunnels. You also have play dates, although they're difficult to set up because the adult has to know the toddler playdate.

The other SPs have multiple gameplay items too, but they each seem to focus on one central item with some minor ones.

Edit: On that note, Kids Room also adds multiple options for kids like puppet theaters and Void Critters.