I have to tout Oregon’s only national park, Crater Lake, which is located in my region. It’s beautiful in all seasons but most accessible in summer. If you’re flying into Portland, you probably will want to take into account four to five hours of driving time to the southern part of the state.
The best summer month to visit most of Oregon is August. However, Southern Oregon is much warmer than other parts of the state with a dry climate and temperatures usually in the 90s, peaking in the 100s, but that makes Crater Lake with its elevation a great place to cool off (although lots of mosquitoes come out at night there).
Southern Oregon also boasts great whitewater rafting on the Rogue River, the Oregon Shakespeare Festival in Ashland, outdoor Britt Festivals concerts in historic Jacksonville, hiking, biking, fishing, wine tasting, and world-famous artisan foods. Enjoy your stay.