Oxford!
I just booked a flight to visit my cousin for 5 days at Oxford in March! I've never left the country before (except for a day-trip to New Brunswick - just over the border of Maine - which was exciting but barely counts) and I am *SO* ruddy excited. It's not ideal timing, what with the wedding coming up, but the RT ticket was only about $350 (though the bus from London is about $50), and your cousin is only at Oxford once, right??