Reem's Quick Pick: If you only have time and money for one-two, I would highly recommend a tour of the Bodlein Library. I would also recommend the official bus tour since it gives a great overview of the entire city in less than an hour while allowing tourists to "hop on and hop off" at designated stops all day on the same ticket, should one want to stop and explore further
Each college has it's own unique charm and history. Although they are not open to the public, many offer their own tours (for a small fee). Others are accessible to students using their student "bod" card - but generally you must present a valid reason for needing to go inside the college (e.g. visiting a particular student or attending an event). The following is a very rough overview list:
Reem's Quick Pick: If you are limited by time, just visit Christ Church College and peek at All Souls College and Trinity College from the views recommended above.
Reem's Quick Pick: If you are limited by time, just visit Christ Church Church Meadows after taking a gander at Christ Church College.
Pensive & Romantic:
Reem's Quick Pick: If you are limited by time, at least meander down Broad Street, Cornmarket Street, High Street, and New College Lane.
Reem's Quick Pick: I haven't tried either of the first two, but I've vaguely heard the church's view is the best.
(in addition to partaking in the above categories)