only just seen this post, hope i'm not to late in answering.
If they are interested in football both manchester united and City have there own tours and museums. I haven't been to city's but united's is very good ( and there is far more trophies to look at:hehe
the museum of manchester is really interesting. I took both my children (6yrs and 15yrs old) last month and it kept us all entertained for hours. If you look online they also do workshops in forenstics and other stuff.
The Trafford centre has a really good cinema, bowling alley, laser quest, and some really nice restaurants and bars. There are some good shops there too but if you want to go shopping the city centre is much better. There is also a huge big wheel in the city if you are feeling brave. (i haven't managed it yet :blush
depending on the weather some of the National trust places are lovely. Dunham massey, lyme park and quarry bank mill are all worth a trip. (but check opening times before you go)
There is also a water park close to the centre of the city which is said to be very good.
And if they are feeling really brave there is the chill factore. close to the trafford centre, it provides taster ski and snow boarding lessons, a luge, tubing and a rock climbing wall. (as well as lots of lovely WARM coffee shops):lol:
Hope they have a good time whatever you decide to do.