You can use the door tool to breathe underwater. If you select it when you're underwater it creates an air bubble for you to step in and out of to breathe! Wicked!
Mine down by digging whilst standing on half a block to keep safe. This will get you to a diamond level super quickly! Woo!
You can avoid getting shot by those scary Guardians by... Simply walking around a post! So simple, but they just can't get you! Genius!
Mow the lawn without even using a lawnmower! Simply place a bucket of water down, then pick it back up again and the water will do the rest for you! Tidy!