The weekend plan
Day one
Start with the colleges most open to visitors — Christ Church, whose grand hall and cathedral are open most days, and Magdalen, whose deer park and riverside walk are worth the entry fee alone. Walk past (or into, when open) the Bodleian Library and the Radcliffe Camera, the round domed reading room that's become the city's defining image. Lunch at the Covered Market, a Georgian-era indoor market still trading daily, then spend the afternoon punting on the River Cherwell — renting a boat is easy even for first-timers, and falling in is a rite of passage, not a disaster.
Day two
The Ashmolean Museum, Britain's oldest public museum, is free to enter and large enough to justify a full morning — its collection runs from Egyptian antiquities to Pre-Raphaelite paintings. Spend the afternoon browsing Blackwell's, the city's landmark bookshop, and walking the Isis (the local name for the Thames as it runs through Oxford) before the trip home. If a college chapel is holding evensong during the visit, it's usually open to the public and free.
What to see
The Bodleian Library's Divinity School, a 15th-century vaulted room that later stood in for the Hogwarts infirmary on screen, is open to visitors as part of a Bodleian tour and is worth booking ahead for. The Sheldonian Theatre, Christopher Wren's first major building, still hosts degree ceremonies and concerts. And the university's Botanic Garden, Britain's oldest, sits quietly next to Magdalen College and rarely has the crowds the colleges do.
A slower alternative: Port Meadow
If the city centre feels too dense, Port Meadow — open common land along the Thames just north of the centre, grazed by free-roaming horses and cattle for over a thousand years — is a 20-minute walk from the colleges and a completely different pace of Oxford.
How to pace it
Don't try to get inside every college — most look similar from the porter's lodge and charge separate entry, and the difference between one grand quad and the next is smaller than it looks on a list. Pick two or three, spend the saved time actually on the river or in the Ashmolean, and let the rest of the colleges stay a backdrop rather than a checklist.