Parenthood or Toddler stuff should be your best calI think.
@LeissaMyst I can't think of many outdoors kid's items in the Stuff packs but Backyard Stuff does have the water slides.
So I see you are looking at Get Together as an EP and yeah it does have the biggest world in terms of the number of lots and it adds social clubs for kids and all other ages as well, but you might want to also consider the Seasons EP. It adds outdoors seasonal activities like roller skating/ice skating, snowball or water-balloon fights, building snowmen (snow pals), trick or treating, plus lots of fun holiday traditions. Heck, kids can amuse themselves just by playing in mud puddles! xD Even if you do go with Get Together for now (it is an awesome pack) you might want to look at Seasons in the future.
Good luck I hope this helps :D
Honestly seasons was a strong second place for my Expansion pack, but the fact is I didn't do much with TS3 version of Seasons so I decided to put it off for another day. Seasonal activities just aren't really my thing tbh; I prefer to wander and roam to/from where I please as far as activities, if I have to fit into a "Pattern" it just gives me a headache; because then I have to make time in my schedule (or my sim's schedules rather) to partake of those events and when they finally do roll around something else may come up. It's better for me to just have a free-form play without the need of going into "timed" events. (Honestly the festivals in City Living are bad enough...)
