Hello and welcome to the RTA forums ! [and Congratulations !!]

'Must See's' are something we don't really do here as we are all different and the road trip is unique to the individual, so what's worth it to you is what's important.

The Grand canyon 'proper' [National park] is more than day trip from Vegas and an overnight really is a must.

To follow your list, it really cries out to do a loop trip starting and ending in either SF or Vegas. It doesn't really matter which being a loop, so check out flight and car rental prices combined and go with the cheapest !

So starting from SF, take a couple of days to drive the coast road and towns along it. From the Cambria/Morro Bay area you can start to head inland [no need to go to LA if you have no real desire] towards the Grand canyon south rim. You will want an overnight stop on route, Laughlin, Kingman or possibly Lake Havasu would be options. From the GC back to Vegas you could detour onto a section of route 66 through Seligman and take in some of the Diners and memorabilia shops on route to Kingman. Hoover dam is anothe rpossible stop. From Vegas you could drive across the spectacular landscape of Death Valley and head North toward Mono Lake on 395, perhaps stopping around Bishop or Mammoth Lakes for the night. You would then take CA120 [The spectacular Tioga pass in to Yosmite] before heading back to San Fran. [via Napa or not?]

That would be quite comfortable with 18 days and possible in 13 or 14 days, but again only you can decide whether to 'expand' your trip or not to include Maui, although I think you will be amazed at the natural beauty and contrasting scenery this route will provide.

A couple of notes. Heading south from SF will put the Ocean side viewpoints on your side of the road and you won't have to cross traffic to stop.

You don't say when in June you are travelling, but it's possible the Tioga pass will be closed due to winter snow accumulation and you may have to go south or north around the mountains. It typically opens late May into June.

Have a look around the forums and planning pages and you wil find much info. Once you have started getting your itinerary in place we can make suggestions and help to 'Fine tune' your trip.