Yes, the Photographer's Gallery will be closed, but if you're wanting photography there is a Muybridge exhibition at the Tate Britain, and Elliot Erwitt at the Magnum print rooms.
I second the Aperture Suggestion, there are a few other interesting camera shops in that area as well.
I always find a walk along the Thames (on the southside) interesting too. From Tower Bridge along past Goldern Hind and the Tate Modern to virtually as far as your feet can take you! There are usually some spectacular views of the Tower Bridge, the London Eye and lots of thing going on to see. I guess you haven't seen the millenium bridge which is along there too.
There is bound to be some event on that weekend so check out Time Out.