Here's a slideshow from SW Portland's little Stephens Creek Nature Park. It's basically a few acres of creek bottomland wedged between SW Bertha Blvd & Capitol Hill Road, with a low bridge over the creek and a couple of other trails winding around the park. Since much of the park is in a ravine below street level, the park seems larger than 3-ish acres, but only so long as you're standing still. If you follow any of the trails you'll be up against the edge of the park in no time, and the illusion ends there, as if you've bumped into the wall of the holodeck or something. The vegetation unit survey for the park (a city report on the environmental condition of the place) lists all the units as either "poor" or "severely degraded". Though that seems to be par for the course for city natural areas. The survey also mentions there's an additional unit to the park, downstream of here, east and on the other side of I-5. It doesn't seem to border any roads and might be inaccessible.
A sign in the park includes an old newspaper article from April 1975 explaining the park's origins. It seems the trail through the present-day park began not as a nature trail, but as one elderly gentleman's guerrilla project. Students walking to or from nearby Wilson High School had to walk along the shoulders of busy streets to get there, and he decided to build an off-street trail as a safer way to get to class. Inevitably, wrangling with City Hall ensued, as it does whenever someone in Portland does an unauthorized good deed. The park sign went on to note that the city eventually signed off on the idea and everyone lived happily ever after, although it tries to be very firm and clear that the 70s were basically the Wild West and we no longer do things that way in this civilized age, so don't even think about it, this means you.
More recently, the city developed a "functional plan" for the park in 2005, with a list of things the city wanted to do fix or improve here, but with no immediate dollars attached. In 2007, the current bridge was added, along with trail improvements. A 2010 story explains that the improvements came about thanks to neighborhood volunteers, who did the construction work themselves instead of waiting for the city to find $900k to have the parks bureau do it. Not really a guerrilla effort, but the earlier thing may have helped inspire this. A bit of trail through the east side of the park opened in March 2014, not long before I visited, after another neighborhood fight over making the trail ADA-accessible.
On May 9th, 1920, the stretch of Bertha Blvd. next to the park was the scene of a high-speed streetcar collision. Apparently the engineer on one of the streetcars ignored a signal and his train collided head on with an oncoming train, killing 8 people (including the engineer) and injuring at least 38 others. A Multnomah Historical Association article about the accident includes a few photos (non-gory) of the scene. The photos show that this area was still rural at the time, and the creek was spanned by a rickety wooden bridge.