My recommendations for visiting Oxford would include a walking tour of some sort. It's almost 10 years since I've been on one, but I think they go inside some of the colleges that you'd otherwise have to sneak into.
The Ashmolean's a good museum, but the Pitt Rivers, dark and packed though it is, is closer to unique; there are plenty of collections of artefacts from here and there in museums around the world.
There's not much to do except admire fantastic architecture and listen to a few related stories, but there are some great pubs too.
There'll soon be a website with talks going on in Oxford: it's in a test version at the moment
(
http://salami.ox.compsoc.net:3000/)
Oxford pulls in a fair few interesting speakers, so you might find something going on.
I'd always book a friend a guest room in college, because it's in a turret overlooking The High, and has a nice feel to it. But if you're staying overnight the hotels are nice: people never seem to complain about the Royal or the Randolph, except the prices.