Let’s say you open a window and there are 20 photos in that window. Want to see a quick slide show of those photos? Just press Command-A to select all the photos, then Control-click on any photo and from the contextual menu that appears, choose slideshow. A full-screen slide show of those photos (complete with a nice smooth dissolve transition) will appear onscreen.
If you want to see only some of the photos in a slide show, instead of selecting all the images, just Command-click on the photos you want in your slide show before Control-clicking on one and choosing slideshow. To quit the slide show in progress, just press the escape key on your keyboard.