One item I always pack is a portable tote. I use it during the trip for hikes, bike rides and for bagging items at a store. On the way home I use it if I have too many items to fit in my suitcase. Last week I used one to carry my hiking boots on the way home from Montana.
The Transit Tote from Matador is my favorite. It rolls up and fits in a small pouch, is made of leak-proof material, and is strong enough to carry my laptop. And it has a side pocket and zips shut.