The museums definitely are your best bet this time of year. Don't miss the Art Institute, of course. Not just for the Wall exhibit, but for everything else. Plus it's next to Millenium Park.
Catherine Edelman Gallery and Stephen Daiter Gallery are across the street from each other in River North, but I am pretty sure that both are between exhibits right now, so I'd call first.
If you aren't tied strictly to photography, the West Loop has some excellent art galleries. Rhona Hoffman would be at the top of my list. Unfortunately, these galleries also don't open their new exhibits until September 7. And I doubt any will be open on Sunday or Monday. Maybe not even on Saturday, since it's the summer.
I like all of the photo stores -- Helix, Calumet and Central -- but I think Central is the only one I'd really recommend to a visitor. It's very cool. But I think it's closed on Sundays, and I'm sure it's closed on Labor Day. Finally, here's a sleeper; International Camera, also in the Loop. It's probably open only on weekdays. It's really a repair shop, but he has a large selection of used cameras for sale.