There's the Ukrainian church with the copper roof overlooking the city in Renfrew. It has a big parking lot beside the church and an unobstructed view of the Stampede Grounds.
http://maps.google.com/?ll=51.058458...,168.9,,0,7.41
It looks like it has a gate on the parking lot, so I don't know if they close it at night, but if they do, the street in front of the church is a cul-de-sac with a sidewalk down to the cliff's edge. It's pretty easy to get to from anywhere in the city too (either Edmonton Trail to 8th Ave or 16th to 6th St).