Lotsa good ideas already by the others here for you.
My suggestions are:
1 - Reverse your circle route to go up/north on PCH & down/south for the Yosemite, Kings Canyon, Sequoyah (that's the correct spelling for this famous Cherokee's name - not Sequoia), etc. This is particularly so if you're wanting to drive it aggressively, because you're on the inside lane of the 2 lane SR1 against the mountain, & you can cross over to the otherside at scenic points.
If in the 914, it's fun to play "double the speed limit" with the yellow diamond signs on the curves!
2 - Below/south of Morro Bay, PCH/SR1 has only bits worth driving, such as Malibu-Zuma-Pt. Huneme, San Clemente to Newport Beach or maybe as far as Huntington Beach & Surfside, & La Jolla/Torrey Pines to Oceanside (if you don't mind the traffic stops every light & tons of traffic. The other OC & LA County Beach Cities cruises on PCH/SR101 are likewise congested but fun for the "eye candy" (see below). These are the old urban Beach City cruises, with the little beach towns now all grown up & sprawled together for a continuum of humanity!
3 - The legendary postcard mountains to the sea Route 1 is north of Morro Bay & broken by San Francisco/Bay Area metropolis - so Los Osos/Morro Bay to Santa Cruz or Half Moon Bay (or into SF), then north of SF to the OR border.
BTW - I strongly disagree with vsg914/Curt, that Morro Bay to SF is anticlimactic, since Big Sur sits right in the middle of that run with the National Park on both sides.
4 - Death Valley in Sept. - plan on 120+ degree days out there even then!
Furnace Creek Inn is aptly named for the temps, & also a nice place to stay.
5 - You might want to consider going "the back way" down US 395 from Tahoe & take the side trips or cross over for Yosemite & the other Sierra spots. Bridgeport, June Lake, Mono Lake (Big Mama Tufa's live there!
), & Mammoth Lakes/Mtn. are all nice. You can still cross over at Lake Isabella at the Southern Sierra NFs' reach.
6 - Can you tell that way back in 70-74 when gas stations gave you stuff to buy gas, that I worked during college doing green marker lines on maps for people who sent in the little 4x5 trip cards from Texaco!!??
So today, you can go to the local AAA & peruse the maps, tour books, etc. & then have them lay you out a killer trip!
7 - Last but not least - remember that it's still Summer in SoCal & inland CA up through Sept./Oct. (except at higher elevations & the far north of the state). So you just may want to reconsider bringing the 914, because after all - THAT is what Porsche obviously intended for us Teeners when the made this promo shot!
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Click to view attachment.
Being a Trojan Alum & a Teener & lover of a good road trip, I'll modify our "Fight On" theme to .....
Drive on!