For all-around stuff and landscapes, I think you just need something 18-xx. It is the kit lens for a reason. 18mm is nice and wide. Not ultra-wide, but decently wide. It's what you want at a minimum for landscapes. 28mm on a crop body like the T2i is not wide at all. The 28-135mm IS USM is a nice lens, but it doesn't belong on a crop body IMO. Especially not as the only lens.
If you want to step up from the 18-55 IS, go for the 17-85 ($450), 18-135 ($450), 18-200 ($600) or 15-85 ($620). Or keep the 18-55 IS and add the 55-250 IS for $250 or the 75-300 for $100-$120 (used). Or Sigma has the 18-125 ($350), 18-250 ($480), or the Tamron 18-270 for $550.
Be aware that the more the zoom (anything beyond, say, 18-85mm) the worse the image quality will be. All lenses are compromises. Zoom lenses are especially so. Something like a 3x or 4x zoom (18-55 = 3x; 55-250 = 4.5x) is generally not a terribly challenging piece of optics to design. Much higher than that, though, and you start to make tough choices. The Tamron 18-270 has an amazing 15x zoom ratio, and this is a startling focal length range to have in a single lens. But the image quality will be quite a bit worse than even the kit 18-55 and the cheapest 75-300.
Of course, we all make that sort of image quality vs. convenience trade-off everyday; or else we'd all be carrying a kit of all primes: 15mm, 17mm, 24mm, 28mm, 35mm, 50mm, 85mm, 100mm, 135mm, 200mm..... prime lenses give the absolute best image quality, but they do so at a very great cost in convenience and versatility. You find few photographers who have an all-prime kit. The question is, how far down the line do you want to go. There are some very good zooms out there, and as I said, if a zoom is 3x or 4x then you generally will still get very good image quality.
EDIT: It would help if we saw the exact deals you're talking about so we can know exactly what you're getting. Links plz.