Versatile lens to compliment the 24-105mm?

[Lorax]

I recently upgraded to the Canon 5D MK II. I had previously bought the Canon EF 24-105mm lens for my Canon XTi knowing I would eventually upgrade. Now that I am using the MK II I need help in starting to find good lens combos.

I chose the EF 24-105 as a everyday, walk-around / travel lens. I am looking for another versatile lens to compliment what I already have. My main interests for the lens are traveling abroad, hiking, landscapes / cityscapes, and as I am a teacher candid school shots.

The Canon EF 70-200mm f/4.0 L IS USM Lens is one that I am leaning toward at the moment as it seems like it covers many of my interests.

Long story short, if anyone has any suggestions on the next lens to add to my collection it would be much appreciated.

[LoveDSLR]

Unless you need f/2.8, the f/4 IS is an excellent lens (I have both and the 5D Mk II). Highly recommended.

However, your next choice could be based on other factors. You may want to consider an UWA, macro, superzoom or a fast prime such as 35 or 50mm as well.

[Lorax]

Thanks LoveDSLR for the comments. I would prefer to have the f/2.8 but am leaning towards the f/4 because of the size and weight difference. I have also considered UWA, macro, superzoom and a fast prime but since all of my older lens are EF-S I am trying to slowly build up my glass arsenal.

Does the 70-200 f/4 IS seem like a good starting point point?