We also availed ourselves of L'Open Tour Bus for our first day in Paris, which gave us a good overview of the city and the more historic landmarks. The downside was in the summer when we went it was a long wait to find space on another bus. Our tickets were purchased before we traveled from Ticketsto: https://www.ticketsto.com/index2.cfm They seem to be a helpful company and have decent customer service.

Happy and Safe travels, Vince & Gerry in Philadelphia