if you could only have two lenses, what would they be? (Zoom & Macro)

IMHO, for your applications, 70-200 f/4L and 100 f/2.8 Macro. The former for sports, the latter for portraits, general use, and of course, macro.

Congrats to your new toy! :)
I would recommend the EF 100/2.8 Macro, razor sharp, relatively cheap (great "bang for the buck"), and works like a charm for both life-size macro work and portraits (at least outside on a crop camera like the XSi). I think the AF is quick enough too for shooting sports even if longer lenses would sometimes be preferrable of course, same for the zoo. But as a compromise if you could only have one lens for the applications you describe, that's my 2 cents...and it's almost twice as long as your 18-55, which should be good enough for the museum, with IS and all.
Good luck!

Aaha you meant 2 lenses other than your kit lens...then I second Steves suggestion with the 70-200 as well..

Low budget:
One lens: Just the 55-250/IS, which is cheap, a long zoom, has a magnification of 0.31x which is pretty big to begin with and which can be enhanced with a Kenko set of tubes or a 250D/500D for macro.

Higher budget:
The same 55-250/IS plus either the EF-s 60/2.8 macro of the 100/2.8.

High budget:
One of the 70-200 L's (The 4's are great and the 2.8's are the fastest but are huge beasts.) or even a 100-400L. These lenses are also great for portraits. The 100-400 is pretty big though and the gap between your 18-55 and a 100 is also pretty big.
Plus the 100/2.8 for macro.

Even higher budget:
Three lenses... (One of the sets above plus a fast prime like a 50/1.4, 85/1.8, 100/2 or 135/2 for portraits.)

Kind regards, Matthijs.

Nikon 14-24 f/2.8
and
Leica Noctilux 50mm f/0.95

:)
EDIT: Sorry did not read the topic to the end :)

zoom: 70-200 2.8 IS
macro: 100 macro

On a budget, I would get the 50mm f/1.8 for portraits.

Real world money:
I have the sigma 70mm f2.8 DG macro and am delighted with it. Nice portrait lens on an APS-C body. Will also work of FF cameras.
70-300mm IS (non DO) a better alternative to the 55-250 IMHO in that it can work on full frame bodies as well. A bit more future proof. Fast AF, metal mount.

Bit more money:
Sigma 70mm DG (can't see past it)
Canon 200mm f2.8 L II A fast focusing sharp ripper of a lens. Not as flexible as a zoom, but this in my experience makes it faster to use. You have one less parameter to think about. Not all that expensive for the quality.

Keeping your budget in mind: For zoo, soccer and portraits with OOF backgrounds in strong light, 70-300 IS (non-DO). For available light and OOF backgrounds in bust-length or longer portraits, the 50mm/1.8 (which turns into an 80mm (35mm equivalent) on your camera These lenses will interface nicely with your 18-55 for your stated uses. Next, I would add a Sigma 30/1.4. For macro, add a good Canon diopter lens to that 50mm.

For the telephoto I agree with the suggestion of the Canon EF-S 55-250 4-5.6 IS. This is an unusually sharp lens for the price range which is in the 'quite reasonable' bracket. For Macro it depends how macro you want to go. Macro officially means a magnification of at least 1:1 or the same size on the sensor as in life. In this bracket the Canon 100 f2.8 macro is very good but the Tamron 90mm f2.8 and the Sigma 105mm 2.8 are close behind.

As an alternartive there are a number of lenses which call themselves 'macro' which should more properly be called 'close focus'. For example the Sigma 24-70 f2.8 Macro doesn't give 1:1 magnification but works quite well for gwetting closeups. Check manufacturers figues on magnification and min focus distance here.

Another option is to also get an extension tube which enables you to get closer to a subject with a normal lens. These come in different lenghts such as 12mm and 25mm. They are just empty tubes with no glass in but which have all the bayonet contacts so the camera and lens work as normal. These are preferable to close up diopter lenses as they don't add to the amount of glass being used.