How long are they in Sheffield for? And do they have any interests? Obvious things which spring to mind are Tramlines (in a month's time), Cliffhanger, usual array of art galleries. Rother Valley is reasonably accessible by public transport and can go sailing and cable tow wakeboarding (if thats their sort of thing). You can get a bus out to Chatsworth now too I think from Sheffield. Leeds less than an hour on the train.