I'd also suggest a monopod (as DreDaze suggested) with a ball head. That way you're carrying something lighter and much less cumbersome than a tripod, but you can rest the majority of the camera/lens's weight on it and just keep the ball joint relatively loose so you can move the camera around.
If you don't really mind variable and smaller apertures, then something like the 55-250 would be great, or you could also consider the Canon 70-300 IS, Canon 28-200, Tamron 18-250, Sigma 70-300, or similar (I'm sure there are other relatively lightweight zooms out there). Of course price and quality between those lenses varies, but you can figure out what you feel like being able to afford and research from there.



