If Religion and Guns are too tame for you ...
hop on to any photography forum and discuss what lenses to take to Africa. Mix in a question about UV filters and you can watch thousands of electrons die terrible deaths.
To try to give a serious, short answer:
-- Assuming that the lens is of decent quality, 70-300mm is a great lens. The "portrait" range is normally considered to be 70-105mm. (This is all calculated full, 35mm frame. I am not a Canon freak, so I don't know if the EOS 1 is full frame. If not (that is, it is a "cropped sensor,") then your 70-300mm is more like a 100-450mm - which is great for portraits and even better for candids in the market and critters in the bush.) Bottom line - excellent choice of lens. (I own a Nikon 70-300mm.)
-- In the old days, I used to use two bodies - one with a 135mm (for candids, portraits, etc.) and a 35mm for wide shots. I tried a 28mm and found that it simply reduced mighty mountains and sweeping vistas to boring nothing. (28mm and wider can be very useful inside buildings, etc.) With a 50mm, on the other hand, I would always have to stop and backup. 35mm was a great compromise.
-- So there are really two ways to go about your question. The first would be to look for a 35mm prime (24mm if cropped sensor) or a mid-zoom that covers the 24 - 70mm range. (I own a 24-70mm Nikon.)
-- The other approach is to go for a "super-zoom" (20-135mm) lens. Problem here is that Canon doesn't really make a good one, and I would not trust to third party lens to be any good over such a wide range. The big reason for using such a lens is that you are always ready and never have to change lenses in the bush, dust, etc. (I own a Nikon 18-200mm.)
Given all that, I would tend to echo Rallyroo and suggest that you go for the fastest mid-zoom you can afford. (That will help with your lower light work.) to which end, get a flash if you can - sometimes wonderful opportunities at night and inside houses.
Some terms:
-- Lens speed means ability to shoot in lower light, as expressed by F stop. (F1.4 is very, very fast; F8, shower.)
-- Shutter speed (fraction of a second) stops action (person running) or camera shake (excited photographer who has just noticed the charging critter). Blurry pictures? Faster shutter/better technique!
-- Stabilization/vibration reduction is a nice feature built into better long (telephoto) lenses which helps to compensate for camera shake; i.e. your unsteady hands. Allows you to use a slower shutter speed - doesn't NOT stop action of running kids, critters, etc.
Rules of thumb:
-- With a lens of 135mm or shorter, you can avoid shake at speeds of 1/125 or faster. (1/60 and slower is possible if you practice, lean against something, etc.) Tripping the shutter is like shooting a gun - smooth, slow, don't jerk or stab, you shouldn't know when the shutter will trip.
-- As noted, when shooting tele, minimum speed should be focal length of lens. That is, 300mm lens, minimum is 1/300. (OK, 1/250) If, considering crop factor, you wanted to argue for 1/500, I would agree. Especially for moving kids, etc.
-- Most lenses are at their sharpest when stopped down two stops. If all else fails, F8 is a good compromise for sharpness and depth of field in "tourist" pictures.
-- Shoot posed portraits at F11 and always focus on the nearest eye. (Assures that the whole face will be very sharp.) If outside with a "busy" backdrop, take a second shot wide open, or only one step down. If you are lucky, this second shot will give you the face in focus and the background blurred.
Finally, buy a good SLR book, "Dummies" or similar. Doesn't cost much, will put you to sleep on the plane, and will let you assess the value of my rants.
N.B. There are a number of pro and semi-pro photographers on this forum. Study their photos and heed their counsel.
Have a good, safe trip!
