Things to do (and see) in Manchester?

I'm going to Manchester this Friday for work and to see some freinds. Besides those things, I thought I might as well stay there for the weekend and just amble about. I dont travel (inside the UK or outside of it) and I'd like to get into the habit of getting more out of the opportunities available to me. I'll be mostly around Salford but I plan to go around, what things stand out to people when it comes to Manchester?