equipment recommendation - How do I select a superzoom lens for travel?

I have my good old Nikon D40. Almost all time I was using default "boxed" 18-55mm lens, but now I feel a big need to buy a lens for getting pictures that I can't get now. I travel a lot and like to take pictures of nature (mountains), architecture (cities) etc. I want to buy one additional lens for such purposes. If it's not possible to combine needs - the priority is a lens with deep zoom.

I'm not selling photos or printing high resolution pictures; I mostly take pictures for my collection.

Price range: up to 600$; Skill level: basic