Open Quick Look in Full Screen: Command+Option+Y

You can instantly open anything in fullscreen Quick Look mode by selecting the item and then hitting the Command+Option+Y keys. If you select multiple files and enter into fullscreen mode, it’s easy to navigate between them by using the arrow keys. You can then exit full screen mode by hitting the escape key.

As a few commenters pointed out, you can also just hit Option+Spacebar to get the same effect.