Just checked the weather now, the forecast is dry so Christ Church Meadow should be good to go (if its raining the natural history museum is still the alternative), here's a list of the places and times:
10.00 Christ Church Meadow (meet at St. Aldates Entrance) (Natural History Museum if raining)
12.00 Radcliffe Camera (meet in front of the vault cafe), drawing around this area to take in Bridge of Sighs, Queens Lane, Sheldonian, Broad Street etc.
15.00 Natural History Museum and Pitt Rivers Museum (meet at front)
18.00 Turf Tavern (King's Arms if the turf is full), end of day meeting point, drinks + sharing sketchbooks.