Hi
I think the one in Castle markets closed ages ago as it was located upstairs near the bridge. Try Hobby Craft but that is in Meadowhall retail park.
how about experience vouchers e.g pampering, zorbing etc.. can be brought cheaply from lastminute.com.
Restaurant vouchers
or how about giving work related stuff e.g start an hour later, finish an hour earlier, excused from a certain task