Cooper Mountain Nature Park - Metro website

"The 231-acre park offers visitors 3 1/2 miles of gravel trails traversing the park's rare habitats and natural features... More than three miles of trails with varying levels of difficulty traverse the park and include a 3/4-mile loop that is designed for accessibility."

Cooper Mountain Nature Park - Tualatin parks website

Trails are gravel, not paved. I haven't been yet.