Use Command+Down Arrow To Open Files

In the Finder, the menu bar shows Command+O to open a file, but Command+down arrow works just as well. It will open the selected file, and also open the selected folder in the Finder and navigate down into it. The benefit is that Command+up arrow will navigate up a level too, so Command and the up and down arrows work nicely together. The exception is in column view where the left and right arrows are used to move into and out of folders.