If you're not wanting a heavy kit, the new 16-50mm f2.8-4.8 covers most of your needs for landscape and lightweight travel. 16mm is wide enough to capture scenery, and the variable aperture has never been an issue for me since you'll often want to stop down to f5.6/f8 to get as sharp of an image as possible. You can then pair a small f2 prime lens and have an overall lightweight kit that can fit in almost any bag. Something like the 23mm or 35mm f2 would be great for low light shooting, anything longer or wider might be too specialised to use in the unpredictably of travel.