The lens that you currently have is what they call "neither-fish-nor-meat". Its not wide enough for landscape and nor does it give a decent telephoto coverage. The answer to your question also depends on what kind of photography are you are interested in. Canon 70-300mm is an excellent pro-sumer telephoto lens and so is Sigma 10-20, a fantastic ultra wide angle. I have a Canon 70-300mm and I can vouch for it.
If you are just starting out with photography, you should go for mid-zoom walk around lens like Sigma 17-70mm or Canon 15-85mm. Both are excellent lenses (with Canon being the expensive one among the two). I own both of them and have never been disappointed with these lenses. They can satisfy almost 95% of your photographic needs on a holiday.
Just to add, you might also want look into few primes including the Canon 50mm 1.8 (aka nifty-fifty). An excellent portrait lens for crop sensors.
Hope this helps
