For more quick photography tips and tricks like this one, click here.
There are many ways to enchance the sky in landscapes. Some people like to boost vibrance or saturation alltogether, while others like to underxpose highlights. Some people like to add GND filters, while others like to use HSL pane or even tone curves for better blues.
Although some of these techniques are quite good and some are outright dangerous and can ruin an image if not used in moderation (HSL panel and curves), the best (and most natural) way to instantly pop a landscape is by tweaking the camera calibration sliders at the very bottom of the develop module.
By boosting the saturation of the blue channel and incorporating that technique in your regular Lightroom workflow, you will enjoy the richer blues of the skies and waters in your landscape shots. Usually a +30 to +50 boost is enough, but you may go to +100 without too much adverse effects in the rest of your picture. On the same note, boosting the greens can be also a great way to get richer grass or forest colors.
The reason why camera calibration works so well is that it directly operates on the RAW file (i.e. all the red, green and blue pixels before demosaicing), while some of the other Lightroom tools work on a rendered picture.
I do recommend you to only tweak the saturation sliders and not the hue sliders, unless you really know what you are doing. It is easy to go overboard so if you wonder why your sky suddenly became purple, don’t say I didn’t warn you :P.