Do you like art? The Manchester Art Gallery has some gorgeous paintings (and is in the city centre), and I would second the Whitworth, which has more of a textile/installation focus.
The Manchester Museum (Natural History/attached to the University) is also good fun if you like to wander round. I love Albert Square (in front of the Town Hall) as a place to sit and people watch, and if it's nice outside, the Oyster Bar (near the Arndale) is a cheap place to get a nice pint in the city centre.
There are loads of excellent restaurants. There's an Armenian restaurant just of Albert Square (down some stairs) that is pretty good and reasonably priced. There is loads to eat in Chinatown (and, actually, a Korean place right near Town Hall (same side of the road as the Armenian one, I think) that is pretty tasty as well.)
The John Rylands Library (Deansgate) is stunning, and they often have cool exhibits on. They also have a nice little cafe attached that is fun to people watch from as it backs on the entrance to the crown courts and, well, sometimes it can get a bit...interesting? when people continue their arguments outside.
Hmmm. I can keep going, if you like, is there any particular food you were interested in?