Depends no what your tastes are, but I'd suggest combining one of the museums with some nosh in the City Centre. May even get a Saturday lunchtime special offer if you catch it in time.
Tri Thai in China Town is nice (try a bubble tea for something different), or you could pop into the Marble Brewery place on Thomas Street and get yourself an authentic Manc pint, and a mega sized scotch egg each for you and the lad (at £2.50 an absolute steal). There are a couple of buffet style places too with an all you can eat thing going on.
AMC Cinema in town is pretty decent and usually quiet and the National Footy Musuem at the URBIS is supposed to be good, not been myself though.
The City Tour has changed quite a bit over the last few years apparently. Went on it last 3 years ago myself, no idea how much it has changed now though.