Which venues do you recommend I make reservations for, and how do I do it?

For Florence, Rome and Pisa to see : Accademia Gallery, home to Florence's most treasured work of art - "Michelangelo's David", the Colosseum in Rome, the Leaning Tower in Pisa and for Valetta, Malta or any other museums/venues you think are really not to be missed.

Thanks for all your help, Helen Ottawa, Canada