We have a nice set of wooden stairs for Shelby to use in the main bedroom - but we didn't want stairs all over the house.
This was our solution - a rectangular yoga bolster that happened to be in the house because I do yoga fairly regularly - we have a couple of them now - one for the sofa and one for the extra bed. Gives her enough height to jump up and makes for a soft landing on the way down.
Rectangular Yoga Bolster | YogaAccessories.com
It also works as a yoga accessory, a cat bed, a dog bed, a floor seat (I like to sit on ground while reading newspaper), and a very comfortable foot stool.