I'd put forward the Sigma 17-70mm f/2.8-4 Macro OS. It's sharper across the frame than the Canon 17-55 f/2.8 IS is at f/2.8 and f/4 (at f/5.6 they're dead even in the centre, and the Sigma is slightly ahead in the far corners) and it has less vignetting and the transmission is a little more accurate to what the f-stop indicates (the Canon is half a stop slower than the f-stop indicates; the Sigma is a third of a stop slower than its f-stop).
It does have slightly more distortion at the wide end, but it has less at the long end. The 'macro' the lens claims is only halfway to macro, and the lens is fairly soft when you're focused that close. But it's better than not having the option at all, I guess. It's about 2/3rds the size, nearly half the weight, and roughly (depends on country) half the price. For an upgrade to the Canon kit lens, specifically for a 'walkaround' lens, it's really hard to make a case for anything above the Sigma 17-70.
Most important in my experience with them, it doesn't have the inconsistent focus of the Sigma 18-35mm. It's not as sharp as that lens is, either, but sharpness means nothing if your image is plain ol' out of focus. Sigma's 'Art' lenses are optically great, but the focus is all over the place with every copy I've ever used, of any of them. (And speaking to other pros and users, it seems to be the standard experience with them.)
If you really need the most light at the long end, Canon's EF-S 17-55 f/2.8 IS is definitely the best choice for a travel/all-rounder zoom on an APS-C body; the Sigma 17-70, despite the more truthful transmission, is still slower at that end of the zoom range. You're getting half a stop less light than you think you are, but half a stop down from f/2.8 is still pretty good. It's also a bit large and quite expensive for an EF-S lens, but it's tough and it'll keep more value if you ever resell it.
If you want lots of light but don't care about the long end of the zoom range, the Sigma 18-35 sure is tempting, if you think you can live with the slightly dubious focusing. Nobody can dispute that it'll get the most light to your sensor, and 18-35 isn't actually too bad a range for general travel and day shooting. I just could never get past the inconsistent focusing (AFMA doesn't help as it's not a consistent front or back focus issue, but both at random; the Sigma dock can only lightly mitigate the issue, too) and the size and weight definitely does make it a little travel un-friendly.
If you do want that longer end of the zoom range and you can live with it being that one stop slower, then the Sigma 17-70 is the way to go; it wins on size by far and it only loses in sharpness to other larger, more limited, more expensive, Sigmas.
Alternatively, there are a few primes worth looking at. If you like that EF-S 24mm, you could take a look at its bigger cousins, the 24mm f/2.8 IS, the 28mm f/2.8 IS, and 35mm f/2 IS, as wlel as the Tamron 35mm f/1.8 VC. They're bigger than the pancake 24, obviously, but still small enough to be balanced on a smaller APS-C body. They're equally as sharp as the EF-S 24, better corrected than it, obviously a little more solidly-built, and they all have IS; obviously those 35mms also gives you an extra stop. (And unlike all the above zooms and the EF-S 24mm pancake, these four primes' transmission actually matches their f-stop, so you're getting all the light you expect.) The Tamron has the added bonus of being fully weather sealed, though that comes at the cost of a little more size and weight. Any prime in the 24-35mm range will work well as an all-purpose travel lens.
Of course, you could also simply add the 40mm f/2.8 pancake lens to the 24mm. Then you've got a standard view (38mm equivalent; close enough to the true standard of 43mm) and a mild telephoto in a pair that takes less space than any other one prime.
It's really hard to recommend things like the Sigma 50-100 as others have done, let alone some of the full Canon L lenses that have been brought up. They're just miserable to travel with, let alone the balance on the smaller body and the reality that they don't actually get you any better image quality or even much better functionality. Go for one of the big Ls if you really want the absolute most rock-solid build quality and full weather sealing and you're willing to accept the price you pay in size and weight for that; if all you're after is better optical quality, the Ls are a huge red herring.
If you want significantly better optical quality than the above small APS-C zooms and primes give and you want to keep to a lower size then it's time to pick up a mirrorless Sony, Panasonic, or Fuji camera.
