Henry P. Leu Gardens - definitely
Lake Eola - definitely
Downtown - definitely
I walked pretty much all of downtown Orlando in a single day back in February 2007. Lots of great architecture. There is the historic train station, City Hall and the Court House. Across from the train station is an old hotel with neat architecture and a covered walkway over the street.
As others stated, further south in Buena Vista are all the Disney complexes. Walk all around the Downtown Disney Marketplace. Some interesting, enormous Lego sculptures over there as well as lots of cormorants around the lake. A good place for general people watching. Beware of the security personnel. They have cameras everywhere. Backpacks and tripods get attention. I was stopped once by a bicycle cop, but he was really nice. He just wanted to know I was a tourist and not a terrorist.