Downtown Ottawa is compact so you can visit the Byward Market, the Parliament Buildings and the Rideau Canal in 3-4 hours. There are some good restaurants in the market area. Be forewarned: as you stand looking over the Rideau Canal you may find yourself there for a length of time watching the locks open and close to allow pleasure boats to pass. It's very busy in July, the weather is usually good and it's a nice way to spend some time. Lunch or tea at the Chateau Laurier on the verandah overlooking the canal might appeal to you.

Another area of the city that has good restaurants is The Glebe.

Considering that you have asked about the Diefenbunker and commented that you'll miss the RCMP Musical Ride, do I detect an interest in military activities? If so, then you might be interested in visiting the Canadian War Museum. http://www.warmuseum.ca/cwm/cwme.asp

A quirky thing to do if you are on Parliament Hill is to visit the area where the homeless cats are taken care of by their custodian. http://www.occdsb.on.ca/~sel/cyberpal/cats.htm

You can tour the Parliament Buildings if this interests you. The next session of government will start on April 3rd and will likely be finished when you visit, unless special bills require a sitting. Aside from the usual security check, visitors can generally sit in the gallery when Parliament is in session. Otherwise, there are tours for visitors. http://www.parl.gc.ca/information/visitors/index-e.asp

One feature that makes Ottawa a beautiful city is the parkland and you are fortunate to be visiting in July. In winter it's a vista of snow and, apparently, the second coldest capital city in the world (Ulan Bator is the first). A claim to fame, I suppose.

Lucy, Toronto