If your interested in human geography, geography is your best bet!
In brief:
Enviro Sci = The science of physical landscape, water, atmosphere, etc. (science) eg. hydrology, environmental chemistry, soil science etc.
Geo = Social aspects of human activity (social science) eg. urban geography, health geography, transportation geography etc,
Environmental studies = The social aspects of environmental changes and how they affect people (social science with some very minor basic science involved), eg. environmental resource management, environmental issues, energy and society etc. (often cross-listed as both ENV SCI/EARTH SCI/GEO)
You also take GIS courses in geo as well.
Geography doesn't offer co-op but you can do experiential education through the social sciences office, which is basically co-op. you can find more information here:
http://www.socsci.mcmaster.c a/EE/
I don't know how popular experiential education is, but I suppose it really depends on the availability of positions throughout the year.
Anyways, good luck on your decision. I'm trying to minor in geo and so far it's pretty awesome! (I kinda wish it was my major, rather than my minor. :p)