If you have a general understanding of, or interest in, weather, I suggest ENVIR SC 1A03 - some people found it difficult, but if you understand it, it's not hard.
ENVIR SC 1G03 isn't difficult, and if you find the material interesting, it's a very enjoyable course, but if you don't care about the difference between limestone and dolomite, you won't enjoy it. Of the three first year environmental science courses, I think it's the easiest.
In all honesty, the best elective I've ever taken is Geography 1HB3. As long as you are competent with maps, it's a very good elective, and I know this is irrelevant to you, but I've found it to be extremely useful for environmental science. There's only one big paper (like there was in ENVIR SC 1B03), and there's a field component, where you and a group make a walking tour of an area of Hamilton.
If you want a course that's not necessarily easy, but is extremely useful, take ENVIR SC 2C03. I did fairly well in it even without 1A03, which is the first year equivalent. This course does teach you a lot about the atmosphere, but it also teaches you how to use Microsoft Excel like an expert.
Basically, my advice is GEOG 1HB3. Dr. Peace is an excellent teacher, the material is interesting, there's a lot of hands on, and as long as you know how to read a map, you should do well.