Drive to Medford, OR on interstate 5. Then follow the road signs to Crater Lake.
Best time there is late Sprint or early summer. The trick is to make sure you don't get there before enough snow has melted to allow access.
This is Oregon's only National Park. It's a bit of a drive from Medford, but well worth it. The island shown is accessible by boat, but getting to the boat is a hike downhill, and a bit more of a hike back up the hill.