Matt talked about the press-and-hold function of the G3X CLR button to quickly get you to the default page. The ENT button has a press-and-hold function called MARK.
When you press-and-hold the ENT key, a popup window allows you to quickly define a user waypoint to MARK your present position. So, for example, if your flying buddy just dead-stick'd his plane into a cow pasture beneath you, you can use this single key to quickly define a user waypoint to MARK the location.
But wait - there is more!
Each time you press-and-hold the ENT key, G3X automatically places a "1" in the Event marker column in the current flight data log entry stored each second to the SD card. So, whether you want to go back and see the lat/lon stored when you pressed the MARK button or just look at the stored engine data when you MARK'd the time when your engine ran rough, the MARK function is very useful. If you are just using it to MARK the flight data log entry, just cancel the pop-up window which offers to create the user waypoint.
OK, one more use for the MARK button. You shouldn't be pressing buttons during takeoff, but if you DID press-and-hold the ENT button just as your airplane lifted off the runway, it will MARK the 1 second log entry created as you lifted off. You can later go back into your log file and see the exact lat/lon on the runway where you became airborne.
Why, you say, would I want to MARK my liftoff point.
Well, if you retrieve the lat/lon of this point from your flight data log file and find the start of the runway by noting in the log file when your logged heading aligned with the runway, you can use one of those online lat/lon to lat/lon distance calculators to see how much runway you used to get airborne.