Recreation junkies can spend a whole summer or a lifetime wandering the trails and peaks of the South Cascades [1]. When you’ve got limited time, the big visits are to Mt. Rainier [2] and Mount St. Helens [3]. Seattle visitors can conceivably drive by both in a single day, as they are only a couple of hours away from one another, but most of that time would be spent in the seated position. Travelers should ideally plan a week at minimum to get out on the trails.
Ideal summertime exploration falls between snowmelt and first snowfall, usually between July and September. Visitors need to be aware of weather conditions prior to heading out. This area of the Cascades typically sees heavy snowfall from as early as October until as late as May, which can be a good or a bad thing depending on your perspective.
Certain roads and attractions are closed, but others open up to a whole new dimension of recreational activity only possible when snowdrifts are present.
Links:
[1] http://www.moon.com/destinations/washington/south-cascades
[2] http://www.moon.com/destinations/washington/south-cascades/mount-rainier-national-park
[3] http://www.moon.com/destinations/washington/south-cascades/mount-st-helens-national-volcanic-monument