My son is 4 and we've bought a teddy from "The Teddy Bear Factory" (the one where you get lots of outfits and shoes and pyjamas and stuff.
I figured he'll want to help do lots of things for the baby, but getting him dressed is one thing he won't be able to do, so while I'm dressing the baby he can dress the bear!
(also it works out cheaper because although I've bought a couple of outfits for the bear, he'll be able to wear the baby's sleepsuits and stuff)