When we lived in Corvallis it was always a nice drive to take the Benton County Tour Route (a national back country byway) through Monroe. We’d head, up to the falls, for some hiking, then head home via Mary’s Peak. In the spring there is foxglove everywhere and blossoming dogwoods. In the fall the colors are amazing with the turning of maples.
Depending on the time of year; the gate may be locked for the day use area. If so, just park on the road and walk in. Alsea Falls is a slide type falls that I like to visit best during the higher early spring flows. After you hiked around and checked it out on the left side, go back to the top and follow the creek upstream to the foot bridge.
On the other side you’ll find the trail to Green Peak Falls. The trail parallels the creek for about 1/2 a mile then veers to the right as it goes through Hubert K. McBee Memorial Park. On the other side of the parking area you’ll see a hiker symbol on a post, this is the trail. It’s around a 1/2 mile to the base of the falls.
References: Waterfalls Northwest