OP, is the Devil's Den area you refer to the place off Leon Creek now called the Leon Creek Greenway?...or somewhere else? As someone else pointed out, San Antonio had lots of off the beaten path places for kids to explore, ride motorcycles, and do whatever. I was born and raised there until college. I got bit by the motorcycle bug in a most unusual way as a kid in the 60's in SA...on a Sears Allstate scooter...LOL!
Friend of mine had a paper route, so those Vespa's, Allstates, and other small wheeled scooters were ideal for the big, newspaper holders to drape over the bulged engine covers without getting tangled in the rear wheel or drive train...plus they didn't require constant clutching. We would go out to the Salado Creek area off W.W. White and Military Drive and ride miles of trails...2-up. My buddy had an older brother who had some kind of brand new, bright yellow, Sears, 350cc single motorcycle that my friend sneaked away one day, and we rode that 2-up on those Salado trails...it might have been made by Benelli...not sure. His brother beat the crap out of him for that...LOL!
I was hooked...had to have a motorcycle. I had worked after school at Handy-Andy grocery store there at W.W. White and Rigsby since I was 14, so I had saved some money. My folks would no let me buy a motorcycle. I left home at 17 and within a year I had a motorcycle. My long path down the evil road of street and dirt bikes continues to this day. Worked part time here in Abilene for many years at a local motorcycle dealership outside of my regular career to pay for all my bad habits. I raced the state's enduro circuit for decades and traveled all over the country on street bikes.
I still ride dirt and street bikes on a regular basis, and I have the Salado trails and a Sears Allstate scooter to thank for it...in a weird way.
Thanks OP for bringing up SA trail memories down memory lane.