Good news!!! Most of the waterfalls in the Columbia River Gorge are very accessible. Many are visible from the highway, or a short walk on a paved trail. A few that immediately come to mind besides the most famous, Multnomah Falls, are Latourell, Wahkeena, Horsetail, and the beautiful tunnel at Oneonta Gorge. Vista House on Crown Point is another must see, and they even have a ramp and elevators if the stairs are a little too much for you (same goes for the lodge at Multnomah Falls).
A visit to Timberline Lodge at Mount Hood is also a must do. Again, easy to maneuver and you literally drive right up to the mountain. Depending on the time of year, you can even ride the chair lift to the top of Palmer Glacier. You can also drive to Trillium Lake, which has a beautiful view of Mount Hood, reflected in the lake, and an pretty flat, easy trail along the east side of the lake if you are so interested.
If you are also planning a trip along the coast, there are some beautiful viewpoints along Highway 101. Cannon Beach and Seaside, the town closes to Portland, also offer some waysides with easy beach access.