Just like Ken said, we need to know the cameras you own to help with specifics,
but,
as a general rule, landscapes are better shot with smaller focal lengths like 20-80mm lenses. If you own a tripod and dont shoot any video you don't need to invest in a lens with image stabilization, but, if you like to shoot video, dont use a tripod frequently or both then its best to look for lenses with this feature. Also, travel is a tricky one. I think you are best going for a zoom lens as it offers the most utility (Especially helpful for travel.. i know, im a travel photographer myself). My main driver is the trusty and widely popular canon 24-105mm IS L. This is a perfect lens for your situation if you shoot canon and is canons cheapest L series lens at around $700USD if you shop around. Most camera manufacturers offer similar lens line ups so look for a lens with IS and a focual length of 20-150ish and you should be fine for landscapes and travel alike.
If your thinking about adding to that camera a good 50mm or 85mm lens will compliment that focal length nicely. and finally to top it off a long telephoto lens, something in the range of 200-400mm and you have covered all your bases.
Goodluck!