Why not fly into LAX? As much as I hate driving in LA, it usually has the cheapest flights and the cheapest car rentals. From there, head up to Sequoia (spectacular and without as much of a crowd as Yosemite or Yellowstone), then Yosemite. Go through Nevada (no map needed, lol), cut through Utah to go to Grand Teton and Yellowstone. From there, get down to Grand Canyon, back to LA and do a day or two at Disneyland. If you watch ANY tv, do a tour at Warner Brothers before going home. Honestly, if you flew into Vegas, I don't know that a DL visit would be worth the detour. Same with San Diego Zoo, and I would love to go back there. And if you're going to detour to LA or San Diego, then how could you skip the CA coastline? But then....
So far, it looks like Las Vegas is going to be cheaper/easier than LAX, especially if I stick with Southwest because they don't go nonstop to anywhere in California from here. But I'm watching fares and car rental prices for both.
I'm kind of thinking San Diego isn't worth the tradeoffs we'd have to make to fit it in, but DLR is a serious must-see - getting to all the Disney parks is on our collective bucket list (for me and both daughters), and this'll be the first time we're anywhere near DLR. There's not much else in LA that caught my eye so far - we really don't watch TV and Disney accounts for an embarrassing portion of our movie-watching habits so the entertainment industry sights don't wow me. Sequoia and Yosemite both look incredible, though, and DD mentioned finding a dark sky park so I'm thinking maybe we'll stay a night near Joshua Tree to do that. Vegas to Joshua Tree to Disneyland doesn't look like a terrible route, though I know SoCal traffic may make parts of it take longer than the mileage suggests it should.
If your plan is to travel through Vegas via Southwest it may be worth taking a look at departing from Bishop FNT rather than Metro DTW. Cuts a tremendous amount of the mishegoss out of flying -- and when you return you will be oh so happy to be headed home within 15 minutes.
San Diego Zoo is special for much more beyond koalas. They also have a Wild Animal Park that is wonderful to explore. Choosing San Diego as part of the itinerary will probably mean cutting some national parks off the list, so I'm not sure how that tradeoff will balance out in the end.
I wish I could make flying out of Flint work - the airport is actually a bit closer to us (in drive time, but prob. a bit further in miles) than the Detroit airport, and it is just so much easier to deal with security and parking there. But Southwest isn't flying out of Bishop any more, and I'd really like to stick with them because of the baggage policies. Paying per bag for an extended trip would add up fast! Plus the only flights out of Bishop now are very regional - I think to get out west, you'd have to change planes in Chicago.
I'm partial to NM since I went to high school there but all of the southwest is beautiful. My son had never been out of Georgia so I was missing "home" one year and took him and his girlfriend on a trip there. We flew into Albuquerque, rented a car and just took off. Had no reservations or plans anywhere, I knew what towns I wanted him to see and that was it. I did buy them a book about hidden places and places off the tourist track to look through before we went so they could see if there was anything they wanted to see. We found our best experiences were just coming upon small places and stopping and exploring. My family did the same thing when I was a teenager. My Daddy had 2 months of vacation he had to use so we took off heading west, with 3 of my brothers (I have 5) myself and Mama and Daddy in the family VW van loaded with camping gear. Back then you could camp pretty much anywhere you wanted as long as it was private land and we would just head out and stop where wanted, pitch the big tent and two of my brothers would pitch little pup tents. We had a ball, went through Arizona and up the coast of California then back across and down. No plans, just going where the road pointed and stopping at interesting places.
I used to love that kind of road trip when I was a kid! I've lived my whole life in Michigan, so for us it was the east coast - the Blue Ridge and Smoky Mountains, upstate New York, one really cool trip to Cape Cod - but back then flights were expensive and camping was cheap, so most of our vacations were loading up the car and getting off the beaten path.
I'm using the fact that we're flying out as an excuse not to camp this time. We might rent cabins at the occasional KOA, something we've done in a half-dozen eastern states without a single bad experience, but there's no practical way to haul a tent and proper camping gear... which is good, because it has been years since I tent camped and I really don't love it any more. I do think I'm going to miss having my own van on this trip, though. I just got window screens for it, and it isn't a bad place to spend a night when we're on the road. But driving cross country would add too much time to the trip - I lucked into three weeks of vacation this year thanks to a change in how we accrue time, but even that isn't long enough to drive from Michigan to California and back.