Californian Road Trip

Can you consider listing your trip by nights rather than the very ambiguous days?

But without looking at your trip forensically, I suggest you consider three nights in Yosemite. And the Yosemite Bug Resort is a beauty, and well worth a visit.

Three "days" between Yosemite NP and LA is reasonable. Personally I would add a night back to San Francisco, and drop one from LA. Personal preference.

3nights SF
2nights Yosemite
3/4nights to drive south
5nights LA

Thanks for the advice! Is this easier?
It's really quite a rough schedule for now, we haven't made any concrete plans yet other than book flights.

I had considered more time in San Francisco, but I've spent a fortnight there before...although my boyfriend hasn't visited yet.
Another reason was we thought the weather would be better south so planned more time there to enjoy the sun (of which we don't get much here in Ireland)

Thanks for your help!

One more thing I should probably add is that we would like to try surfing.
We took basic beginner lessons in Bali and enjoyed it a lot. Where would be a good beach for beginner surf lessons even just for one day?

3nights SF
2nights Yosemite
3/4nights to drive south
5nights LA

I would still suggest you give yourself two full days (three nights) at Yosemite NP ... it really is one of the very great national parks in the US, and worth the time. Take one night from the drive south.

Getting from Yosemite (the west side) to LA, I would stay in Monterey and either San Luis Obispo or Santa Barbara. If you can squeeze one more night, as per the above suggestion, then consider both of those.

I know next to nothing about surfing lessons on the Southern California coast, but I do understand the water is really cold. It is NOT like Bali - or Australia, for that matter. So you either need to give it a miss, or invest in a decent wetsuit.

The brochures really do lie!

3days to drive south (Carmel/Monterey/Big Sur/Santa Barbara)

First, the bad news:
Recent storms have destroyed two sections of Highway 1, one in the north and one in the south (see map here ). In the north, a bridge was damaged beyond repair and has top be replaced. The new bridge will not be ready until the end of September, barring delays. In the south, the road was covered and destroyed by a landslide, and will not reopen until sometime next year.
Much of "Big Sur" is simply off limits. You can drive as far south as Bixby Bridge to get a feel for it, but you'll have to drive back north and detour to US 101 to drive farther south. There is a section open, via the Nacimiento-Fergusson Road, but this is slow, narrow, and dangerous road, which may be closed at times for military exercises (it passes through a base) or to allow construction equipment through (to repair Highway 1).

We've travelled a lot here in Europe and in Asia where youth hostels can be almost like budget hotels with some character, 'flashpacker hostels or boutique hostels' I've heard them described as. From my googling this doesn't seem to be a thing in California.

Hostels aren't as common in the US as elsewhere in the world, although they are more common along this stretch of the country than anywhere else.

Our budget will be hopefully less than €180/night so I guess this will leave us in pretty basic motels.

~$200 per night for basic? Only if you want it to be. That budget will get you well more than "basic" in most places.
Watch out in San Francisco, however. Large conventions can take up most of the rooms, and drive up the prices.

Third and final question is regarding our car rental! Is about $500 a decent price to pay for this or what should we expect.

That's about 41 and change per day. That sounds about right for a one-way rental. (Sometimes there is a one-way fee, other times the fee is hidden as a higher daily rate.)
Does the rate include mandatory insurance?

We enjoy [...] street and casual food and drinks

Try to plan to be San Luis Obispo on a Thursday night, for the Thursday night market.

I like to visit less touristy places
would like to fit in a few hikes or outdoor exploring days

Steal one of your LA days and spend a day in the Channel Islands.

Please take a close look at the map, most of the scenic coast is part of the detour, you'll miss a lot.

Consider a couple nights down to Monterey then head to Yosemite, 2 nights by-in the Valley and then through the top on Hwy.120 to Lee Vining or Mammoth Lake, stunning drive and scenery. Next drive to LA down Hwy.395.
Any idea where you think you'll stay in LA?

Another option:
3 Nights SF
2 Nights - Monterery/Carmel
2 Nights Yosemite
1 Night - Lee Vining or Mammoth Lakes
5 Nights LA

Hermosa and Manhattan are nice beach communities just south of LAX.
You can rent wet suites at most surf shops, north Manhattan Beach and El Porto are good begging surf spots, depending on conditions. The ocean will be chilly but bearable, especially for the Irish!

Fun bicycle riding up/down the beaches, good bars, restaurants and shops. Laid back entertainment.

Huntington Beach is also nice but crowded and far south. The south bay beaches are much better for relaxing and less crowded surfing.

$200us a night for lodging will get you by.

I would look into doing a tour of Warner Bros studio instead of, or in addition to, Universal. $65 for a 2.5 hour tour in small groups of about 12-15, you can get off and walk around the various sets and studios and ask questions of your guide, unlike Universal which is a giant tram with 200+ people on it. Warner Bros also has some Harry Potter stuff.

Hermosa Joe beat me to the suggestion of South Bay beaches, there is a hostel in Hermosa Beach, Surf City hostel, which at least has private rooms (I do not think the one in Santa Monica does). Sea View Inn is also nice but may be out of your budget that time of year (Manhattan Beach). A block from the beach with free parking (rare) and use of beach chairs and bicycles.

Hopefully you have lodging reservations for Yosemite, it books up well in advance although some people report being able to get a cancelled reservation by calling.

I don't think Huntington Beach is too out of the way. Depending on where you are staying, if you hop on the 405 Freeway in the early morning, you will be traveling south. Most of the heavy traffic is going in the opposite direction. There are several well regarded companies that provide surfing lessons, and I find the sand and water a bit cleaner than the west facing beaches near LAX. Avoid weekends to avoid the crowds.

Mid September to mid-October is when San Francisco and the Central Coast actually have their best weather. Warmest and most likely to be fog free. Highly unlikely to have rain. "Warmest" in San Francisco is a relative term. Although I have been there when it was 38, 22-25 would be more likely for a high.

You can get surfing lessons in the Santa Cruz area, south of SF, but be aware that the water is cold enough that surfers usually wear wetsuits. They often wear wetsuits in the LA area as well. A surf school will have them for rent.

You can drive to Big Sur a bit south of the Bixby Creek Bridge, but you will indeed have to turn around and detour north, unless you get very lucky and the new bridge opens early. This web site shows what is open and accessible. Scroll down to the second map where the closed highway is marked in yellow. The closure at Lucia will probably be lifted by Sept. but you'd still have to take the Nacimiento Fergusson Road, which, as mentioned, is not for the faint hearted.

It is currently possible to hike around the closed bridge, but there is no transportation south of it, and no parking near the trailhead. There is a shuttle from a parking lot to the trailhead and people are doing it as a day trip, but it's mainly used by locals (kids have to walk it to get to school!)

Be sure you have your lodging in or near Yosemite booked as soon as possible. Even in Sept. it can book up well in advance. There are hostels in Monterey and San Luis Obispo, as well as several in SF. The Yosemite Bug is part hostel & part resort. (I'm not familiar with hostels south of San Luis.)

You won't need that car rental until you leave SF, so that will save some money. There is good transit; plenty of taxis, Uber, and Lyft; and it's surprisingly walkable. Parking can be hard to find and pretty expensive. When you price rentals, check both airport sites and getting the car in town (many rental places will deliver the car to you). There are a bunch of airport fees and taxes that drive up the price. On the other hand, it may be easier to rent without a one-way drop off fee if you rent airport to airport, SFO to LAX.