Perhaps something similar to a Baracuta G9? If you don’t feel like paying the price for an original, then places like Lands’ End and L.L. Bean Signature have harrington-styled jackets at lower price points in a variety of weights.
Still, I’d suggest an unlined, unconstructed linen blazer.
ADDING from dieworkwear:
How about a M65 or a field jacket? Not an actual M65, though I guess you could, but some designer inspired piece.
Can you tell I’m not the best when it comes to thinking about casualwear? Definitely check out military surplus stores. I’ve got a tropical poplin coat that I wear sometimes. Pretty lightweight stuff. You can nab one at Army-Navy Sales.
ADDING from funkypresident:
how about a 3/4 trench, which can also be good for rain? A Barbour Bedale is very versatile, and while I don’t typically associate Barbours with the west coast (it evokes Brit country or east coast prep to me), but that could be its charm.
If you can afford it, then definitely. Maybe a bit heavy for the warmer weather though? I’m not entirely sure. I’ve only tried one on once and that was during the winter. Amazing coat, but was totally out of my price range.