One lens to cover the majority of your needs: EF-S 15-85mm IS USM.
Not sure what you consider "affordable".... the 15-85mm is a $700 lens. Top image quality, good build, fast focus, helpful IS and a nice wide to moderate tele range of focal lengths... all in one lens.
Your stated subjects - "farms, open landscapes, small houses, rivers and spectacular sunsets... cities with skyscrapers" - all are just asking for something wider and really don't call for a telephoto. So I don't know why people are recommending 18-200mm lenses, except it's relatively cheap. 18-135 isn't any wider than what you now have, either.
The only thing the 15-85mm lacks is low light capabilities, so you might want to tuck another lens in your pocket. Depending upon whether you want a wider view or a tighter portrait lens or macro capabilities, you might choose might be a 24/2.8 IS, 28/1.8, 50/1.4, Tamron 60/2 macro/portrait or 85/1.8. For portraits the inexpensive 50/1.8 might be fine, but it's focus isn't good in low light situations. The Sigma 30/1.4 is another possibility, but it's comparatively large and heavy, and I've seen some reports it also struggles to focus in low light.
The f2.8 "walkaround" zooms are relatively large, heavy and expensive, plus all have a moderately narrow range of focal lengths, little different or less than your 18-55 offers now. f2.8 might be helpful for low light, but one of the smaller prime lenses will be one or two stops faster.