Another cool feature about Android is the ability to enter various numerical codes to the phone app to unlock various statistics and other bits of heavy data. This archived Reddit thread lists all the currently (as of writing) known codes, but the following snippet only lists codes for Samsung and HTC devices. Your mileage may vary.
List of Android "Secret" Codes, Tips, and Tricks. from Android
Sometimes you just want a little more granular control over your phone, but you don’t want to flash the software to something like Cyanogenmod since the current version doesn’t work well on your phone. For times like this, the best thing to do is enable Android developer settings on your device.
I like these settings because they let me set the phone up just so, and add or take away things that I’d like to see in the UI. The Android team hasn’t given us every possible option yet, but there are a lot of great options such as turning off animations and other little tweaks to make the OS run a bit faster as well as nice at-a-glance changes like adding a numerical battery percentage to the charge indicator.
To enable developer options, open the Settings page by clicking the cog icon in the notification shade. Tap on “About phone” to bring up the appropriate page, then tap “Build number” repeatedly until Android congratulates you on your persistence and awards you the coveted Developer Options menu.
Additionally, Marshmallow includes another secret menu called the System UI Tuner that allows for minor tweaks and adjustments to the interface–all in the name of customization! (Take that, Apple!) To access the System UI Tuner, pull down the notification shade and long-press the cog icon until it begins to spin. You’ll receive a notification and can then access the new menu option from the Settings panel.
One word of caution: The System UI Tuner mostly contains experimental options that could change, break, or disappear entirely when your phone updates, so keep that in mind. Meanwhile, enjoy!