I'll second Phil's suggestions. If visiting MOSI, do take in the world's oldest railway station (Liverpool Road), which is part of it. Opposite the entrance to MOSI (and again part of it) is the Air and Space Museum, should planes be your thing.
If time is on your hands, the Imperial War Museum North is about 2 miles W of the city centre in Trafford Park. Speaking of which, the Rugby League Final (Warrington v Leeds) is at Old Trafford, k.o. 6pm Saturday night, so it may get a bit congested round there, and in the city centre pubs afterwards, particularly if Warrington win.
And speaking of pubs, the Briton's Protection (high Victorian) adjacent to the Bridgewater Hall on the corner of Lower Mosley and Gt Bridgewater Streets is worth a visit. Find your way to the back of the bar, where it's less crowded
http://www.manchesterbars.com/britonsprotection.htm. As an alternative to Chinatown, their Grunt, Gobble and ZoomCoo pie might fill you up.
Note: the museums are free.
Andy