Manchester is famous for its football, isn't? Manchester city and Manchester United are the two teams related to the city and visiting their stadiums is a must if you go to Manchester!.
There are also tons of stores, the main street Market St, is full of starbucks,Mcdondals,Primark,Topshop,etc. Picadilly Gardens or the city hall are another two stops you should do. This city is too small and you could see almost everything in just one day, truth be said, is not that beautiful compared to another UK cities such as Liverpool, York, Newcastle ... but its magic is to get lost in its streets, come across with different kind of vintage shops in Afflecks (my favorite building in the city :P) and just relax in any of the gardens you have in city centre. I really love this city because when you are there you have that feeling that you have tons of things to do and it's always full of people (nightclubs here are amazing!) and think is a great option if you want to go to the UK to study english or something like that.
City Hall looks like a Hogwarts building, it's worth it a visit!!
John Rylands Library is another must !, these pics are from its inside and looks like Hogwarts itself!
The printworks is another great building you should visit! everything is really close to city centre
This is the football museum and it's free !! , you have here tons of activities to do and different games. Also you can take a photo with the champions cup
This is the Britannia Hotel, good option to stay the night in city centre