A couple places near my neighboarhood (Villages at Riverdale), are the Adams County Regional Fairgrounds and Eastlake.

Both are nice since they have established trails and lakes.  Parking is plentiful anytime at Adams County Fairgrounds.

On really nice days, during peak hours, parking can be tough to find around Eastlake, but you can usually find a spot to street park. 

There's also a really big off-leash dog park in Westminster, near Standley lake.  It's not fully enclosed, so your dog shoudl respond to voice command or stay on-leash.

For a nice on-leash dog park, there's one out in Louisville, not far from the Colorado Tech Center.   

Barr Lake state park is another nice place to walk a dog, though there is a day-use-fee, unless you ave a state parks pass.