In reply to iusedtoclimb:
Pitt Rivers is a must. If you ask a child to describe a mountain, you get the Matterhorn; if you ask them to describe a museum, you get the Pitt Rivers - shrunken heads, canoes, mummies, totem poles...
Ashmolean as well, if they are into museums. It's now so different to the PR that it might not be overload - where the PR is extremely old-fashioned, the new Ashmolean is very modern. Very good (and usually quiet) restaurant on top.
Climb St Mary's tower for good views of the centre. I'm not sure paying to go into a college is really worth it; have a look around the (free) Old Bodleian if you feel the need for a quad.
G'n'D's is a great place for lunch - great bagels and ice cream. They're on Little Clarendon Street, and there's a branch near Christchurch too.
Punting, if it's a nice day? Hire from the centre, or (perhaps nicer) the Cherwell Boathouse on Bardwell Road.
Annoyingly the OU pool is membership only.