Any advice on what to do in Manchester, UK?

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In September I will have 4 days all to myself in Manchester. I have booked myself a nice little hotel in the northern quarter and wont have a car. Any advice on what to do that I can reach on foot? Is the metro easy to use? I am on a limited budget but open to most suggestions. Not very interested in sports though and I love to go bargain hunting in thrift/second hand stores. Any advice greatly appreciated