On May 26, June 30, July 28, Aug 25 (all Saturdays), there will be bluegrass jams at Collier Memorial Park (logging and early history museum). This is on Hwy 97, about 30 miles north of Klamath Falls and about 40 miles south of Crater Lake.
Haven't been to this before, myself (even though I live about 5 miles away), but I'll go this weekend and let you know how many people are there and what it is like.
There is a great state park/camping spot across the street, set right at the confluence of two famous flyfishing rivers - it's a nice, relatively inexpensive, has hot showers, hookups and a separate tent camping area. A little further down the road is a "primitive" Forest Service campground.
"Post" jam, there is Crater Lake to explore and Diamond Lake, Lake of the Woods, Fish Lake, Rogue River and Klamath Lake to camp, boat, birdwatch or fish. Also Hart Mountain Wildlife Refuge (antelope) for some rugged backcountry camping.