Try going down to the trails next to the GM Head Office (I believe it is on Wentworth or Wilson, whichever runs east - west) in the spring. You will find E. Garter Snakes, Painted Turtle, and maybe even a Red-eared Slider (non-native turtle). Thats all I was able to find the one summer I was working in Oshawa, but I'm sure there is a lot more there (I only looked on my lunch breaks).
Pelee Island is an excellent trip. You'll see some great herps only found on Pelee and meet a lot of great herpers. Jeff and Jenny do a fantastic job of organizing these trips and the food is fantastic.
Heres a good start for information about herps found in Ontario:
Canadian Amphibian & Reptile Conservation Network
and maybe pick up a field guide for Reptiles & Amphibian of North America (Eastern/Central)