Are you going for spring break, or in the summer? Arizona in the summer is HOT. The higher elevations (Flagstaff, Sedona) aren't so bad, but still --
Spring break is another thing entirely. Perfectly fine weather that time of year.
I'd +1 on the London trip. I can also recommend Buenos Aires. That's an awesome city. It's a beast to get there, but once you're there you have all the liveliness of Latin America with the glamour of Paris, at discount prices.
"You'd be surprised at how much it costs to look this cheap." -- Dolly Parton