Great selection of books and magazine with a big focus on rare. out-of-print books Japanese photography. Can sends hours in this little shop. I'd try to avoid on Market day (Saturday) if you want to have a good look as it can get a bit busy for such a small space.
The walk along the South Bank from Westminster to Tower Bridge will take you past a plethora or interesting sights and cultural hotspots. There's always something interesting going on too.