An UWA (ultra wide angle) would be useful for architecture/buildings, for sure...
when I visited DC and NYC I only had my 17-70mm lens for wide angles and it just wasn't wide enough when shooting large buildings (outside, mainly...though, the UWA is good inside as well).
I've since acquired a Tokina UWA, 11-16mm and it is plenty wide, now...the only problem is, my wife won't let me go back to NYC, again! heheh...but we'll probably visit DC again next Spring.
We also shot Chicago all up (a couple years ago) and that was with the 17-70 since I didn't have the 11-16 then...but Chi-town is only a few hours away, so a guy could visit there pretty much whenever he wanted 
A tripod is pretty much a 'must have' if you're doing any indoor shots...outside, as long as it's daytime you can get away with out one...and, in NYC they don't like seeing any tripods on the streets--way too much foot traffic in the Big Apple!
One thing to keep in mind, also, when using wide/UWA lenses, if you point the camera up or down you will have a lot of distortion on your buildings...at times, it can't be helped (especially, in NYC) since the buildings are so tall and you can't back up far enough to shoot with the camera being level...heheh
If you leave enough 'room' on the edges/sides of your frame/composition you can then 'straighten' up the buildings in post-processing (with PS/CS, etc.)...and if you don't leave that extra room at the sides/edges, when you do straighten up your building/s you will end up cutting them off at the sides/edges!
My 'shootin' partner, Bob, shoots wide angle shots with his Canon 10-22mm (I think it goes to 22mm)...I shoot Nikon, so I'm not as familiar with Canon lenses.
He likes his 10-22mm...we've gone downtown in Detroit and shot lots of architecture in the Motor City (but it aint nothin like shootin NYC and DC or Chicago--Detroit is dinky/crappy compared to those towns!)
Good luck and have fun in your new photographic adventures, and we'll be looking for those pics to be posted here, when ya get back with the 'goods' 
Jim K.