Sitting here in the office with the sun shining outside, contemplating an ideal day in Oxford, my mind is so full of fun possibilities that it's hard to imagine cramming them into sixteen or so hours, especially given that one of the nicest things you can do round here is spend an afternoon lying on the grass in your park of choice with a bottle of Cava and a cryptic crossword. Assuming you've got limited time in the city and actually want to do some stuff, I'd suggest the following:
Rise early. Work up an appetite with a stroll (or a run) round Christ Church meadow, hopefully while the mist is still hanging over the water, and look out for crazy people rowing instead of sleeping (or strolling), then breakfast at the Grand Café on the High Street. It's arguably somewhat overpriced, but offers low-key elegance in terms of menu and decor, with a nice line in lethal coffee. Having hopefully survived the coffee, pop into a couple of nearby colleges (clap at the mound in New College and graze with the deer in Magdalen), before continuing on a round of sightseeing. Other people have recommended the Pitt Rivers Museum on this page, and they're not wrong - it's wonderful - also the Bodleian Library, the Covered Market, Blackwell's Bookshop, St Mary's Church, Modern Art Oxford, and of course lots of the other colleges, are well worth visiting. Consider trying a walking tour - a good way of absorbing lots of historical titbits, taken with a pinch of salt. Stop off at intervals for lunch in the garden of the Vaults and Garden café on Radcliffe Square, or a pint at the Bear, the Turf or the Eagle and Child. Finish up by wearing your legs out for good and all at the fabulous new-look Ashmolean, working your way up to the roof terrace for a well-deserved round of tea and cake.
Now go home, relax - then put on your sparkly eye make up and go out for dinner in East Oxford at the Magdalen Arms, (or try Fishers, Aziz, Door 74). Yes, they're right about Moya too. Eat lots - you've earned the calories. For the evening's entertainment, I'm not really qualified to recommend gigs or clubs; after such a busy day I'd rather split the evening between a couple of favourite pubs: If you're a bar person I'd try Kazbar on Cowley Road, but I'd probably usually recommend the Fir Tree on Iffley Road and the the Half Moon on St Clements, where you'll hopefully come across a lively folk music night.
Stumble home at two in the morning singing snatches of traditional ballads and vowing to take up the fiddle. Sleep.