When the sun is out, Oxford is arguably the world's nicest place to be. Yes, there are no beaches, but we do have two glorious rivers. Note that the River Thames is the cleanest river in the world that flows through a major city, so you can even have a dip if it gets really warm. If river dipping is your cup of tea, I would recommend that you head to Port Meadow, bring some sandwiches and chill next to the banks. Delightful.
There are also a huge range of Balls & Fundraisers, Cinemas, Classes & Clubs: Sports & Fitness, Music & Dance, concerts, Courses & Workshops, Exhibitions, Festivals, Fairs and Markets, Gardens, Gigs & Comedy (includes open mic nights), Lectures, Seminars & Conferences, Pub & Club Events and Theatres - head to http://www.dailyinfo.co.uk/whatson/ for more info.