Oldham has great links into Manchester and the M60 will get you out of there easily
If you're going to the Arena the IMAX cinema, Hard Rock Cafe and lots of other chainy eating places at the Printworks are nearby.
Chinatown
Manchester Museum
CBBC tour at the BBC studios at MediaCity could be combined with the Imperial war Museum or the Manchester United tour
Jump Nation at Trafford Park and also Legoland Discovery and Sealife at the Trafford Centre (M60).
Chill Factore is nearby
If you have time Eureka in Halifax is not too far from Oldham and right next to the train station.
Also the Media museum in Bradford is easy from there (and free)
Heaton Park for picnics. There's a little farm and a tram museum
Go Ape at Rivington or Delamere Forest
M60/M6/M56 makes it really easy to get across to Cheshire if you want outdoorsy things - Tatton Park, Applejacks, Stockley Farm, Quarry Bank Mill near the airport, Alderley Edge, Jodrell Bank, Dunham Massey, Red House Farm, Chester, Chester Zoo, Liverpool Docks, Knowsley Safari Park.
Manchester Airport visitor park
No need to be bored in these parts 