Silver Falls State Park is Oregon's largest state park with over 9,000 acres. This area likely has the greatest concentration of scenic waterfalls in the U.S. located in an easily accessible area. The famous Trail of Ten Falls runs eight miles, but one can see nine of these falls by driving to three different locations and hiking only about three total miles.
Most impressive are the North Falls and South Falls which are short walks. Surprisingly, you won't not find mosquitos to be a problem. The only downside could be crowds if you visit on a weekend.