The Portland  Memory Garden is not a trail, but is fully wheelchair accessible and sounds like a lovely outdoor experience. 

Where: Off SE Powell at 104th at the southeast corner of SE Bush and 104th.

"Part of the Portland Parks system (i.e. Ed Benedict Community Park), this garden was designed especially for people with memory issues such as dementia, Alzheimer's disease, head injuries and those who are physically frail -- the garden is fully wheelchair accessible and the design of the garden is such that folks can get very close the plantings in the raised beds enabling them to use their senses to experience them.  It is safe and secure for those in walkers or wheelchairs--wide pathways that are solid and level, lots of shade or sun areas and four-seasons of lovely botanical diversity to enjoy." - Patty Cassidy, President, Friends of the Portland Memory Garden