You will find that there are TONS of accessible sights along the Historic Columbia Gorge Highway, including Multnomah Falls. There is an elevator in the historic lodge there and a wheelchair ramp to the main viewing area. The trail is paved all the way to the bridge, but it might be a little steep for someone with limited mobility.
Other waterfalls easily accessible are Latourell, Wahkeena, and Horsetail Falls. All of those areas are paved, and easily seen within a short distance from the road.
Also, make sure to stop at Vista House at Crown Point. There is a wheelchair ramp into the building, and an elevator inside to access the lower floor.