Um, don't go there at all?
I went to high school in Medford, so maybe I just know the area a little too well!
Crater Lake is totally beautiful and worth seeing. I don't know how outdoorsy you are, but there are a couple of things worth doing - taking the boat to Wizard Island or hiking around the rim. And of course you have to picnic somewhere where you have a pretty view!
If you have time at all, stop by Ashland and take in a play at the Oregon Shakespeare Festival, particularly one in the outdoor traditional theater. You can order tix in advance or get up way early and sit in line for them. While you're there, wander around Lithia Park and all the hippie shops.