http://www.themaroller.com/
Back in the day everyone who shopped at natural food stores got one of these sooner or later. Basically the same thing as the tennis/laX balls, but a lot easier to control (and a lot harder when you really need to dig in, but not too hard if you control the amount of weight you put on it).
The other thing my PT started me on is segmental rotation. Sit crosswise in a straight chair and put your hands one on each side of the chair back, but don't use them to pull. Keep your hips planted and rotate your upper body toward the chair back, first from the low back, then the mid back, then the upper back. Do it three times, then switch sides and do the other side.
It takes practice before you can feel and gain control over which vertebrae are rotating, but this is a great exercise.
Then there's good ol' cat/cow. You can "roll" the apex of the arch to target different areas of the spine.