Enjoy your trip.
The National Football Museum is worth a visit if you're in Manchester City Centre. .
As for gift shops, the museum is within walking distance of the Arndale Centre, where I believe a new City shop recently opened. The club shop at the stadium is always mobbed on matchday and best avoided unless you really want something and are on a tight schedule. Who wants to be carrying shopping bags at a match anyway?
One tip I will give you is to remember to buy a few extra pin badges. They're relatively cheap but I guarantee you'll bump into some football-mad kid back home and a gift of one of those will make his day.