I lived for fourteen years (1973-1987) near the intersection of 23rd Avenue and Monaco Boulevard in Denver, which is about a mile west-southwest from the threshold for Stapleton's runway 8R. I developed my love of airliners from watching the regular stream of arrivals on 8L and 8R, or departures on 26L/R, from my house.
In terms of proximity to aircraft movements, Stapleton was a pretty good spotters' airport. In addition to the aforementioned observer's theater (which closed in the late 1980s, well before the airport itself closed), the airfield had a number of good perimeter spots, particularly along the south side of the airfield at the approach end to 26L. The southern perimeter fence lay a mere 500 feet from 26L; on summer days you could smell the burning rubber as the aircraft touched down. The only other perimeter spot that's closer (that I know of) are the spots along the south side of Miami International. The Miami spots, however, are on the opposite side of a roadway, allowing traffic to obstruct your view. Not so at Stapleton; a number of parking lots abutted the actual perimter fence, meaning only the chain-link obstructed your view. In fact, one entrepreneur had constructed a wooden observation platform high enough to view over the fence--which the city closed after security concerns arose.
The east-west runways protuded to the east of the rest of the airfield, furthermore, such that Havana street curved around the thresholds, affording the spotter a number of good spots underneath and on the northside of the approaches as well. I even passed the time under the actual flight path of the arrivals on 26L/R, until the perimeter patrol got in the habit of moving along those who parked there.
As noted above, Stapleton was a major station for three large carriers in the 1980s--United, Continental and Frontier. All three maintained large maintenance facilities and flight kitchens. United's hanger and ramp, furthermore, abutted Quebec street. One could drive along Quebec Street, and the tails of parked UA aircraft would overhang the road. I recall seeing a variety of UA aircraft, including a number of DC-8s (-62s I believe) that UA parked for some time after their retirement.
Of course, United still maintains its pilot training facility on the grounds of Stapleton at Quebec Street and Martin Luther King Jr. Boulevard.
What Stapleton lacked, unfortunately, was a variety of traffic. The vast majority of movements consisted of UA, FL and CO aircraft, with regular if infrequent interruptions from the other majors. It could be frustrating. For a long time Stapleton had no regular B747 movements; for a while UA had seasonal Honolulu service with B747s, but I think that ended in the late 70s or early 80s.
Stapleton lacked international traffic as well, aside from Mexicana's B727s. The only intercontinental service, until Martinair's abortive Amsterdam service of the 1990s, was Condor's weekly charter to/from Germany. I always enjoyed the polished aluminum of Condor's DC-10s when they arrived. If you didn't catch them at the perimeter, you could snap photos from the terminal's ice cream shop, whose windows overlooked Stapleton's sole international gate (C-2).
Stapleton was the field at which I developed my love for and knowledge of commercial aircraft. While DIA is an architectural marvel, and finally has traffic volumes surpassing Stapleton at its height, I miss Stapleton's spotter-friendly layout.