Been there, done that for the National Archives last September. Fun place to photograph. On Sundays, there no one so it's all peace and quiet.
Recommended: make some nightime shots of downtown Ottawa, really nice and full of possibilities.
Be obvious: tripod, filters, lightmeter, camera, Big Camera, Really Big Camera that scream "Hey, I'm a photographer for chriss' sake" and you'll probably have your mugshot taken by some of those three of four letter agencies.
That happened to me and that the reason that I'm a good boy since then
You can take the Ottawa skyline from Gatineau.
The Byward Fish Market
The School of Photographic Arts located at the same place (on my list of places to visit next time I'm there)
At last, the photog museum is open (it wasn't last year, grrrrr) so you'll have an intersting place for you to go.
National Museum of Transportation may be interesting also.
Also the steam train ride to Wakefield
Really, it's so full of possibilities that it won't be a problem for those creative juices to start flowing
Have a nice trip !
Cheers
Marc