Denver has gone to the dogs! It has many designated dog parks, which are fenced and maintained. To maximize your experience at a dog park, here are some tips:
Invest in a class or hire a trainer who will help you and your pup develop a bond of trust, respect and communication. If your dog is reliable on basic commands, especially "come", any outing to an off-leash park will be a pleasure. Do you really want to chase your dog around a dog park, yelling and pleading with it to come to you?
Enter the off-leash area with your dog on a leash, and observe the activity around you. Is your dog frightened by lots of commotion? Often, there are lots of dogs "play fighting", which can be scary to some dogs. Don't hesitate to leave if you feel uneasy; perhaps a small play group is a better choice.
Don't assume that every dog you encounter is friendly. A wagging tail doesn't always indicate an approachable dog. Learn to recognize signs of stress and arousal in another dog's body language, which could equal trouble for you and your dog: high, stiffly wagging tail, staring eyes, low growling, curling lips. Walk slowly away with your dog in tow.
Bring a copy of your dog's health record, and keep it in your car. In an emergency, it will be handy,plus it has written proof of vaccinations (which should be current, or documented by your vet that your dog has built up immunity and doesn't require vaccines). Make sure your dog is licensed - if there is a problem involving your dog, you want to be "legal."
There are some links that will help you locate a park near you:
Denver area - http://www.denvergov.org/Default.aspx?alias=www.denvergov.org/PRDogsOffLeash
Parker area - http://www.douglas.co.us/publicworks/parksandtrails/Bayou_Gulch_Regional_Park.html