Actually you can take the sightseeing bus(double deck), which will take you through most of the sight seeing sites in the city of Ottawa. You can purchase the ticket in downtown Ottawa, or go and ask tourist information office (located in front of Parliament Hill). As far as I know, parking is not free in most places.
If you plan stay more than one day, Gatineau park is a must see, but you need to drive there. If you come with friends, you can also visit Phillipe lake, where you can BBQ, picnic, swim and play beach volleyball.
