Doors Open Ottawa
This event is lots of fun, especially on foot and in good weather. Some of the sites are good for a short self-directed visit, others have guided tours. For non-downtown sites you'll want to plan your route, otherwise most of your day will be spend driving.
You can get stuck on a long tour at a boring site. Unless someone has recommended it, I'd steer clear of the tour-offering sites for that reason. Other sites are always open to the public, so going on the Door's Open weekend will just mean they're extra crowded. Best to avoid them.
Architecture: Christian Science Building at 288 Metcalfe St.
Architecture: Dominion-Chalmers United Church at 355 Cooper St.
Architecture: Army Ottawa Officers' Mess at 149 Somerset St. W.
Architecture: Knox Presbyterian Church at 120 Lisgar St.
Long Tour: Water Treatment Plant (either Lemieux Island or Mud Lake, they're basically the same)
Exhibits: Government Conference Centre at 2 Rideau St.
Architecture: Russian Orthodox Church at 99 Stonehurst Ave.
Short Tour: Northern Art Glass at 955 Gladstone Ave.
Long Tour: Traffic Operations at 175 Loretta Ave. N.
Exhibits: Supreme Court of Canada at 301 Wellington St.