You mention that you're looking for something with zoom, and @dpollitt gives some logical recommendations there. Be aware, though, that excepting the most expensive suggestion, these are all quite slow variable aperture zooms, and debates about zoom-vs-prime image quality aside, will generally not share a lot of handling characteristics with your 50mm. So, if you really like using your 50mm and want something to complement it, you may want to consider a prime lens of a different focal length.

It's true that the particular lens you have is an amazing bargain and there aren't other offerings quite like it. However, Canon offers a full range of lower-range fast prime lenses (in addition to very expensive models which we won't discuss here). Of these, several are the logical next step:

• Canon EF 24mm f/2.8 ($360)
• Canon EF 28mm f/2.8 ($250)
• Canon EF 35mm f/2 ($330)
• Canon EF 85mm f/1.8 USM ($400)

Like the 50mm, these lenses are generally well-regarded for the price. 35mm/50mm/85mm completes a classic trilogy of prime lenses — a typical "complete set" before zoom was common.

On a 1.6× crop-factor SLR like the Canon 550D, the 50mm acts as a short portrait prime (as I think you've noticed, from your description). The 85mm is in the same general class, but for tighter shots (for example, portraits featuring just the face). You could also use it for some of those landscape details (although it is not a super-telephoto by any means).

The 35mm is closer to the "classic normal" field of view of a 50mm on a film or full-frame camera. And 28mm is actually even closer to the "true normal", which is equal to the diagonal measurement of your camera's sensor. (In addition to the f/2.8 version I've linked above, for about twice the price there's also the Canon EF 28mm f/1.8 USM, which I'd also seriously consider.)

The 24mm gives you another classic favorite field of view, and is also well regarded for its price and size/weight. And, as the widest of the lenses I've listed, it might best fit your desire to fit more into your photographs. You can get lenses that go wider than that (24mm on 1.6× is not very wide angle), but price-for-image-quality starts to go up exponentially.