Jefferson Park in the Mt. Jefferson Wilderness Area is a great hike. Real popular in the summer, especially on weekends.
Several trails provide access to Jefferson Park. Breitenbush trail head at Breitenbush Lake(rough road) and Whitewater trail head (good road) are two of the more popular starting points.
Quickest route into the Park is probably from Whitewater trail head -- about 5 or 6 miles one way. Uphill going in, downhill coming out. Here's a link to an article describing the Whitewater trail head approach:
The article at the above link gives driving directions to reach the Whitewater trail head from Salem (about 70 miles, with the last 7 miles on a Forest Service road).
Salem is about 50 miles south of Portland, 60 miles or so from Portland International Airport. Total drive time from Portland would probably be about 2 or 2 1/2 hours to the trail head.
There's a Forest Service campground (maybe two of them) on the Santiam River along the highway near the FS road that goes to the Whitewater trail head. So, if driving from Portland later in the day, might want to camp at the FS campground and have a short drive to the Whitewater trail head to start hiking the next morning.
From the Breitenbush trail head, it's about 3000' going in, then over Park Ridge and 1000' down to the Park, so that's 1000' to go back up on the way out. Here's a pic from Park Ridge looking down at Jefferson Park and across to Mt. Jefferson, after making the appoach via Breitenbush trail head.
A two night, three day trip would be to hike into the park on day #1 from either Breitenbush or Whitewater, spend day #2 roaming around the park (several lakes for fishing), then hike out on the third day.
Here's a link to a photo of the park from Park Ridge, approaching from Breitenbush:
Here's a link to a photo of Mt. Jefferson from the Park in July 2010 -- looks like there was snow in the Park itself at the time:
Another option for hiking into Jefferson Park is the Pamelia Lake trail head (good road) which provides a much longer, roundabout route to the Park by linking up with the PCT at Pamelia Lake (which is 2 miles from its trail head) and then following the PCT into the Park. Pamelia Lake trail head is a longer drive from Portland, maybe a half hour or so past the road to the Whitewater trail head.
The Pamelia Lake trail head does provide another option for a 3 day loop trip that doesn't include Jefferson Park. The Pamelia Lake trail leads to a trail to Hanks Lake and Hunts Cove on the south side of Mt. Jefferson (opposite side from Jefferson Park). Here's a link to an article with map (click on "cameras" for photos) describing that loop option: