If you have an Excel document with multiple sheets, Quick Look will let you view all of them. A bar at the bottom of the window, but inside the "QuickLook" area, shows the name of each sheet.

It isn't immediately apparent, because the QuickLook arrows don't show up to let you flip through the sheets; look just above that area, however, to see the sheet selection bar, from which you can choose a sheet with a mouse click.