Downtown there's a fenced in dog park, just on Assiniboine between Garry and Donald, but it's small, so I'd avoid that one. Kilcona park in NK is huge, no fence, but last time I was there, not a lot of dogs and very open, so you can see them coming from far away and change your course to avoid them. For a reactive dog, you may have better luck at the Charleswood one. It's about 80% fenced, only opening other than a handful of gates is on the west side by the curling club. To get the dog used to other dogs and people, keep them on leash and walk around the outside of the fence. Your dog will be able to see the other dogs, people, but still be on the other side of the fence.