For example making different headline levels appear in different font sizes or changing the color scheme.
There are lots of ways to customize
org-mode. Many of the
color-themes for Emacs (e.g., Zenburn, Monokai) include customizations for the colors and sizes. You can also use the
org-bullets package to add pretty bullets to each level.
In addition to the detailed tweaking that you can do following Ryan's suggestions, you can quickly try doing
M-x customize-themes and check
leuven -- it is a good light colour theme with different heading sizes and code blocks highlighted. Another good option is https://github.com/jonnay/org-beautify-theme