I haven't been in Italy in August so I can't tell you about the necessity of booking in advance for all of the sites.

However, in Florence, you might want to book for the Galleria dell'Accademia http://www.polomuseale.firenze.it/english/musei/accademia/

In Valletta, Malta, you must book if you want to see the Hypogeum. Only 10 people are allowed in for each time slot. http://www.heritagemaltashop.com/hm/site.aspx?locId=51

You might instead go to see Tarxien temples which I don't think need advance booking. When we were there, a group from a cruise ship was visiting. http://www.heritagemalta.org/visiting.html

Frances Toronto, Canada