Here are my thoughts. There is no right or wrong to what you're asking.
Mid May is great convertible weather where I live. Oregon and Washington, maybe not so much. You do have a top on your car so I would pick May or mid to late September. Between those months it's hot where I live in Nor Cal.
I would go north to south for the reasons Enblethen said. You can pull off to take pictures without crossing the highway lane. Not that that is a big deal in a car.
What to see is up to you. Lots of beautiful scenery along the way. Lots of glass shops and wood shops if your into that type of thing.
There is a lot of good web sites to punch into to get things to see. If you like weird things like I do this is for sure my favorite.
Roadside America.When your between Eureka Ca and Garberville Ca be sure to get off of 101 and go onto the "Avenue of the Giants." Super, super pretty in there. It would be even better in a convertible.
Good luck, have fun and be safe.
BTW great car you're doing this in. Very nice!
~ Too many freaks & not enough circuses ~
"Life is not tried ~ it is merely survived ~ if you're standing
outside the fire"
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