With 3 full weeks I would think about flying to Salt Lake City and renting the car there. From SLC you could see several National Parks (Arches, Canyonlands, Capitol Reef, Bryce Canyon and Zion).
See the South Rim of Grand Canyon and then head back to Salt Lake City to turn in your rental car. You can fly or ride the train from SLC to San Francisco.
The key is to avoid a hefty drop off fee on the rental car.
Try to return the rental car to the same city as where you rent it. There could still be some snow at the higher elevations in late April. Perhaps delay for a week or 2 if you can.

I don't know which would be more expensive, one way drop off or having to fly from SLC to SF.

With three weeks, you do have plenty of time to explore the beautiful parks in Utah before heading to the Grand Canyon and California.

You could fly into either Salt Lake, Utah or Grand Junction, Co and visit Arches, Canyonlands, Capitol Reef, Bryce Canyon, Zion, South Rim of Grand Canyon and head to Yosemite before making it to San Francisco.

It's so hard to say when you will hit good weather or bad weather in the spring. We were in Yosemite in early March this year and hit the most beautiful weather for three days. Days were warm and sunny and then a week later they had to close the park because snow knocked out electricity! It's been a very cold and dreary May this year in California but no telling how it will be next year. We also spent 10 days in Zion National Park in mid April and hit more wonderful weather.

Utahtea

The south Rim of the Grand Canyon is a 10hour drive to SLC. You don't need to go to Salt Lake City.

The two closest major airports to the Grand Canyon are Phoenix and Las Vegas. The next best would be to fly into LAX or San Diego. Spend a couple of days in Southern California and then drive to AZ/the Grand Canyon.

But then you have a problem. In April you will NOT be able to drive the scenic route from Arizona/Nevada to Yosemite. The road across the Sierra (Tioga Pass) will not be open (it is still several weeks from opening this year, but this snow season has been pretty exceptional). Getting to Yosemite from the west side would not be a problem.

So what to do?? A good option would be to spend a week+ hitting the Grand Canyon, Zion, and Vegas. Then drop off the car and fly to either San Francisco/San Jose/Oakland. Then you can rent a different car and visit Yosemite and maybe some of the California coast before hitting San Francisco and flying home.

"Brian is not very good with cold"
So he tells me as i am replying to all who have been kind enough to answer. Can you suggest a better time of the year. I have taken note of flight suggestions.
thanks for prompt replies

Janisj is right. If you only want to go to Grand Canyon, go to Las Vegas and rent a cheap car. I suggested SLC with the first night at Moab which is much closer than the 10 hours from SLC to the South Rim of the Grand Canyon. With 3 weeks, you can see a great deal of the SW United States.

"I suggested SLC with the first night at Moab which is much closer than the 10 hours from SLC to the South Rim of the Grand Canyon."

But Tom -- you then suggested going back to SLC. One way or the other you are talking hundreds of unnecessary miles and at least 3 extra days of driving.

I would not go to Salt Lake City. Too far, too cold in April.

janisj has a good itinerary outlined there for you

aussie3: "Can you suggest a better time of the year."

There are some issues when picking a date. The Grand Canyon/LV are in the dessert southwest and Spring is a wonderful time to visit. By June they are getting very hot.

Spring is also great for Yosemite Valley which is the iconic bit. The waterfalls are torrents then. But you cannot get into the back country or the the vast majority of the park as a whole.

The problem w/picking a date is we don't know WHEN Tioga Pass opens -- every year is different. Sometimes it opens by early to mid-May and sometimes not til late June. There are fodorites visiting here right now who assumed when planning their dream trip months ago that they'd see LV, Death Valley, Yosemite and maybe Lake Tahoe. And that isn't possible w/o driving hundreds of miles out of the way.

April can still be quite cool in the mountains (or it can even snow) - or it can be lovely. May gives you a better chance but no guarantees. Normally folks can water ski on Lake Tahoe on our big Memorial Day holiday weekend (last weekend) - but this year they were snow skiing.

Early June may give you the best combination of good weather -- but by June both the Grand Canyon and Yosemite will be getting crowded.

didn't mean to hit post . .

What I was trying to say -- it is pretty much a crapshoot. What ever dates you pick, you can work out a nice itinerary. But for someone who doesn't want cool/cold weather -- Aprilis probably too early

If you want to miss the crowds and the summer heat, you might think of our fall like mid-Sept. to mid-Oct. Tioga Pass is open, the crowds have left and the temps are very nice. The ONLY draw back is the waterfalls in Yosemite could be dried up. Spring is the best time to see the waterfalls if that's a must see.

Utahtea

Sept to mid october? We are very adaptable and want to see the best we can for this one off experience. Water falls arnt that important.[Yosemite.]
Is it a really bad fee for dropping a car at a different city?
We are use to long drives.Try Australian adventures! Nothing is close here in the West.
But are happy to follow advice and fly in to San Fran and go to Yosemite from there if that is better.
As all we know about the Grand Canyon is its name so hearing about different drives etc is great.
We looked at doing a camping water adventure but the costs are a bit staggering. Thou they sound wonderful re scenery and we love doing this sort of thing as a alternative to just driving.
Also staying in the Canyon at different interesting places.
Am I expecting too much?

The fee for dropping off a rental car in another city can be quite high ... as much as $700 in some cases. That's why everyone is suggesting various combinations of driving and flying to you. And I second those suggestions. I'd do the parks in Utah plus the Grand Canyon and Las Vegas in one leg and then fly to SF or Oakland and rent a car for a round-trip to Yosemite from there. That's the best use of your time and money in my opinion.

If you really wanted to do a lot of California driving, you could fly from LV to Los Angeles and do a round-trip drive to San Francisco and Yosemite from there, but you might feel rushed.

If I had three weeks, I'd devote 10 to 12 days to do the Utah--LV--Grand Canyon Route. I'd devote the rest of the trip to SF and Yosemite (about a week should do it). That gives you a little leeway to see if there's anything else you might want to do. Even then you won't be able to see everything.

But I do agree that September/October is a better time. If you go in April, the north rim of the Grand Canyon will still be closed, so you can only visit the south rim, a longer drive from both Las Vegas and Utah. (The North Rim also closes in October for the winter, though the date changes annually.)

But by October we are often getting snow in the Sierras. Not usually heavy but it willnot be warm.

If warm weather and water sports are important to you - then the window wouldbe June - mid September. But during that period it will be HOT (yes even hotter than OZ) in the southwest. (Except this year -- there was a blizzard w/ white out conditions in the Sierra last night -- very VERY unseasonable weather)

That is why I said its a crapshoot. The best time in one part of your itinerary will be a not-the-best time for other parts.

OH - to be clear, my post probably should have read this way:

>>If warm weather and water sports are important to you - then the window would be June - mid September. (Except this year -- there was a blizzard w/ white out conditions in the Sierra last night -- very VERY unseasonable weather)

But June - Sept will be HOT (yes even hotter than OZ) in the southwest.<<

Thanks to janis j and doug for your follow ups.I think we are now going for mid september and taking your advice and splitting the trip. Car and then fly, then car . Ending back in San Fran.
I have been reading all the post for the area and will take advice and try and book in parks and also look for best area to drive and maybe do a boat trip.
Do you think its worth the cost of long boat trip and forget the car drive thru Canyon or not?
If you had the choice which area would you concentrate in?

"and forget the car drive thru Canyon" Do you mean the Grand Canyon? There is no road/driving through the GC.

Just one point -- just to confuse you more . . .

In Sept you can take the scenic route across the Sierras (Tioga Pass) so you could drive from the Grand Canyon/Vegas over the mounntains to Yosemite/San Francisco.

You may still want to >>Car and then fly, then car<< to save on the drop off fees. But just so you know all your options . . .

What boat trip are you referring to? OZ to CA? Rafting down the Colorado river thru Grand Canyon? My mind is full of possibilities.

If you return your car in Utah or Las Vegas you may save drop-off charges but be sure to compare that to air fare before deciding.