Oxford is a picturesque town full of history and impressive architecture at every turn. Just an hour's train ride from London, it's a perfect day trip destination. The town is super walkable, with helpful street maps on every other corner. Make a plan before you go to decide which colleges you want to visit as most require an entrance fee of a few pounds or more (it adds up!)
The £7 entrance charge to Christ Church College is worth it. The campus is lovely, from the elaborate cathedral to the wide courtyard and, best of all, the awe inspiring great hall which inspired the dining hall in the Harry Potter films. Best to visit Christ Church in the morning or afternoon as the great hall is often closed at lunchtime so students can eat!