Zoph can create a slideshow out of any set of photos: an album, a category, all the photos by a certain person or even search results. Simply click on "slideshow" whenever you have a set of photos on your screen.
This means, you can easily make a slideshow from only the good holiday pictures. Simply go to the search screen, select the album "Summer 2009" and Rating greater or equal 8, click on search and when it shows the result, click on "slideshow".