Mono Cliffs Provincial Park has some excellent hiking. It includes a small lake (really more of a good-sized pond) called McCarston's Lake, which can be hiked to:
(Picture from Flickr user jbcurio, used under Creative Commons License.) There are also various smaller streams and ponds running through the park.
There are two things, however, that may not make this a good answer to your question. One is that although this is not "west of Guelph", it is well north of it (and Brampton as well.) If you're coming from the Niagara area, then it might be a bit of a drive.
The other caveat is that it's a provincial park, and so there's a vehicle entry fee for the front entrance. However, when I visited in May 2013, it was possible to park along the back side of the park and enter the trail system on foot. If you consult the park map, the road you want is marked "2nd Line EHS". Nominally, I believe you're supposed to pay a $2/person walk-in fee; however, I don't recall there being any way to actually pay it at that entrance.