I'm finding a separate "pet room" is good - keeps traffic down around dishes and/or cat boxes. Also, the tv must be in a separate room from the pet beds - I got sick of one simdog howling at it all the time.
Also, I'm finding a separate homework area is good - I kept having problems with adult sims getting stuck by a bunch of simkids sitting around on the floor LOL.
And a greenhouse! I love the gardening part of Seasons and have even started connecting them to the houses, then using the greenhouse roof over the sim kitchen, too. Ahh. Now if only Eaxis of Eavil had included herbs in their pie menu of things to plant.
With the BUY I've been able to cut down on the ratio of bathrooms to bedrooms. Helps a lot. I can now easily do 2 bedrooms/1 bath or 3 bedrooms/2 baths (which is a lot more realistic to me...but then I'm not into realism, just space-saving).