Canon EF 55-250 IS lens is the ideal compliment. This lens will not fit on a full frame digital EOS if you decide to go that way in the future.
For your present camera it ticks a lot of boxes:
Picks up where your 18-55 kit lens stops.
Has very good reach, especially with the x1.6 crop factor of your camera.
Has useful image stabiliser, you may get away without a tripod in many situations.
Is a good performer optically.
Is keenly priced (theres nothing better for the money)
It is a light lens so no hassle to carry round all day.
On the downside:
Its a bit lightweight in terms of construction
The front filter thread rotates, making the use of grad & polariser filters a bit trickier.
As said, it won't fit full frame EOS cameras.
Check out www.photozone.de for comprehensive reviews of this, and other canon and third party lenses.