Although it's not marketed that way, Canon's EF 24-105mm f/4L IS zoom lens is not terrible for portraits.
Especially on the older pro bodies with the APS-H sensors, the 1.3X crop factor means your effective focal length range of 31-136mm encloses the trinity of standard portrait focal lengths: 85mm, 105mm, and 135mm. (Here's me lamenting again that Canon abandoned the APS-H sensor size with the launch of the EOS-1D X.)
While f/4 isn't a screamingly fast aperture, it's big enough to get a reasonable amount of subject separation from the background. You're not going to blow the surroundings out in a creamy blur of bokeh, but at the same time you won't be worrying as much about not having enough depth of field to get both eyes in focus, either, so there's that.
