How much you wanna spend?
the
canon 28-105 F3.5-4.5 USM (not the crappy one that starts at F4.0) can be had for a good price (though brand new copies are getting harder to find).
It takes quite sharp photos and is reasonably fast. Zoom range is perfect for a full frame camera like the 5D (its a little long for a 1.6crop camera like I have but it is still my go-to lens)
If you want to spend some money, the "L" series lenses in this range (the 24-70L and the 24-105L) are both good zoom ranges for every day use. The 24-70L is faster and can have shallower depth of field but the 24-105L can zoom a little more and has image stabilization which can partially make up for the slower speed.
Both of those will run you a LOT more money than a gently used 24-105 3.5-4.5 USM (I know it gets confusing that they have at least 3 different lenses with a 24-105 range). I would suggest you just do that until you figure out what you like. Then, when you want to buy a nice L lens, you will be able to either buy the *awesome* 70-200 f2.8 (if you decide you want to go long) or the 16-35 f2.8 (if you want to go wider)