If you're busy exploring everything that OS X El Capitan has to offer, one of the smaller new features that you might not have come across yet is the ability to automatically hide the menu bar from view — if you want a truly full-screen experience for your regular apps, it's easy to toggle it on and off.
As you could probably predict, if you know your way around Apple's desktop software, the setting is under the General heading in System Preferences. Check the box marked Automatically hide and show the menu bar and you're good to go — the menu bar magically disappears whenever the cursor isn't heading for it.
For a completely clutter-free experience you'll want to automatically hide the dock, too, and as before you can access this setting through the Dock page of System Preferences. It's particularly useful if you're planning on taking advantage of El Capitan's Split View feature, giving you more room for your web browser, spreadsheet application, or whatever else.