Worcester College and Lincoln College are free to visit (Worcester has beautiful grounds) , Trinity charges a small fee, New College and Magdalen are larger colleges which charge a slightly larger but still reasonable fee. Christ Church is the biggest and the most expensive.
For an evening meal - try the Bear, or the Royal Oak on Woodstock Road, or Carluccios on Little Clarendon Street. For lunch, have a wander around the Covered Market and if the weather is nice buy a picnic to take down to Christchurch meadow (next to Christchurch College, but free to access).
Have a look also at the excellent Ashmolean Museum, the delightfully quirky Pitt Rivers Museum (an ethnographical museum containing a Totem pole, shrunken heads from the Amazon and a witch in a bottle) and the display of rare manuscripts at the Weston Library - all of these are free to enter.