1.Reply to Phone Calls with Canned Messages

This is one of those features that we all know exists, but many of us forget because we don't tend to explore the UI when we're receiving a phone call. However, most Android phones have the ability to send a ready-made text message to the person calling you when you can't talk. In stock Android, you can do this by swiping up when you receive a call, instead of to the left or right. Then you just pick any one of the pre-made messages and your phone will automatically reply with that message to the person calling.