If you have an AMOLED screen, by just setting black background as wallpaper, you can extend the phone battery life and can use the phone for more time.
Speed up charging:
If you are running out of time and want to charge your phone fast, just turn on the airplane mode. Airplane mode will disable all the wireless communications and charge your phone much faster.
Speed up the phone:
You can drastically speed up the phone just by reducing the animation time. To do this, go to the settings, about phone and tap on build number version 7 times, this will unlock the developer options. Then it's just a click away in developer options to reduce the animation time to 0.5x
Reduce Data usage:
If you are worried about the mobile data limit being exceeded, you can simply go to settings and disable background data. There will be N number of apps that collects the background data and sending to the server. Disabling it will simply save your mobile data.
Fake your location:
Once you turn on developer options, there is an option called mock location app and by installing fake GPS app you can fake your location to anywhere in the world. At least you can fake your friends by posting the location in Facebook, WhatsApp etc.
In developer options, you can check all the running background services and see which apps are consuming a lot of memory. You can uninstall the apps that are using more RAM even in the background. These apps are poorly written by developers, uninstalling them and installing alternate apps will make your device much faster.
Don't keep activities:
The name itself is self-explanatory. The app or activity will be killed automatically whenever you leave the app. Just go to developer options, to enable this setting.
Just hover your camera on the mathematical equation and the app will solve the solution of the complex problem and immediately display the results.
Sleep Cycle app:
The app basically tracks and analyzes your sleep and wake you up by automatically from the lightest phase of sleep.
Using custom launcher:
Using a custom launcher like Nova or Apex launcher will help you to set more gesture based options. For example on my phone, I used swipe up to launch WhatsApp, swipe down to show notifications, double tap to lock the screen.