One of the most enjoyable places to walk, bike, or rollerblade in downtown Portland is along the Willamette River. Both the Eastbank Esplande and Waterfront Park on the west bank bank offer smooth, wide trails with great views of downtown and of Portland's Bridges.
For a great three mile loop, start anywhere and cross The Hawthorne Bridge at one end and and the Steele Bridge at the other. For something a bit longer, the Eastbank now connects to the Springwater Corridor, a 16.5 mile, paved, and mostly off street trail that follows the Willamette to Oaksbottom Park and then heads east towards Boring, Oregon.