I'm going to be going to Leeds and Glastonbury festival this year (as I did last year) as an Oxfam steward. I can't recommend it enough, you meet a great bunch of people, and everyone you meet is going to be friendly to someone with an Oxfam Tabard on (and it makes it really to blag free booze).
Anyway, Oxfam are looking for people to steward and campaign for them. Stewarding involves a range of tasks, I have had - operating gates, running a Barbecue, driving around Glastonbury delivering coffee to people. If could be anything.
Campaigning basically means walking around the festivals with a clipboard and encouraging people to sign up for newsletters. You work 3 shifts, but will have plenty of time to see lots of bands, and take in the festival atmosphere.
Details
here.