I like bardo's list, but I think I would do New England instead of New Orleans on this trip (New Orleans is quite far off your route). I'd consider the following:

San Francisco: 5-6 nights (which includes a couple of day trips outside the city, perhaps Napa, or Carmel)
Chicago: 3 nights
New York: 5-6 nights
Washington, DC: 5 nights
Boston: 3 night
Other New England: 3-4 nights

Here are links to a few mini-reports I did --- one when we did a circle trip over 4 days to and from New Haven, CT, another a day trip we took from our home, which is just south of San Francisco, and the last, of our 5 days in New Orleans last spring:

http://www.fodors.com/community/unit...ew-england.cfm

http://www.fodors.com/community/unit...art-way.cfm?28

http://www.fodors.com/community/unit...nd-more.cfm?13

Many thanks to everyone for your very helpful responses. We are still digesting them. We have decided to visit in Sept/Oct and also take the advice from Littleman and this time visit only the east coast. We can leave the west coast for next time. It makes sense.
Many asked for our interests. We love to walk when visiting other countries and to soak up the atmosphere. We do not enjoy long organised tour group travels but enjoy short orientation tours of cities, etc. We enjoy big parks and gardens and landmark monuments. Also the cuisine and wine of the region. We are not really museum people although some are "a must". We like to see good stage shows and simply taking in the scenery and architecture of the regions we visit.
We would like a 3 night stopover in Hawaii on our return journey. Does anyone know if you can fly from the east coast?

Continental Airlines has a direct flight from Newark (NYC) to Honolulu, I'm sure others do as well. I have a brother in Aus and I'm in Hawaii and unfortunately stopovers are not the deal they used to be. it is sometimes cheaper to fly in and out of LA, then get a second ticket for a side trip to Honolulu.

I would suggest that you play around on www.kayak.com to price out flights. Try different multi-citiy routings. eg. Australia-DC to NYC-Honolulu, or the reverse, then try SYD-New york-DC-Honolulu. I often find that simply reversing the order, using different gateway cities, or of course changing the dates, you can often save a huge amount of money when doing these long haul trips.

OK, all east coast changes things a bit (not for better or worse, just different). With 4 weeks, I'd consider:

Boston - 3 days, then rent a car and drive around...
New England - 3 days, then either drive or train to....
New York - 6 days, then train to....
Philadelphia - 2 days, then train to....
Washington, D.C. - 5 days, then fly to...
Charleston - 2 days, then drive to...
Savannah - 2 days, then fly to....
New Orleans - 4 days

(there are scads of other alternatives....you could stay in just the northeast for the whole month, adding in upstate NY, Niagara Falls, maybe any of Toronto, Montreal and/or Quebec, more time in New England, etc. You could spend time in Virginia (Charlottesville) around the DC portion of your trip. Asheville, NC. St. Augustine. Miami. So many choices!)

You have received many really great suggestions thus far. However, most of these are urban areas. Most of America is rural... you may want to explore some areas outside of certain cities.

New York - you are a bus or train ride away from a very beautiful area, the Hudson Valley. You can easily spend 2-3 days there and explore historical mansions, wineries, sail on the mighty Hudson River, hike the beautiful Shawangunk Ridge.

San Francisco - you many want to rent a car and ride down Route 1 (Big Sur) or go north to visit the giant Sequoias or Red Wood forests... it is absolutely unique. Sonoma Valley is also a nice day ride from the city.

Las Vegas - unless you are absolutely passionate about shopping, eating, drinking, gambling, and would prefer to forego sleeping, spend only a day there and head out to some Zyon National Park and Bryce or the Grand Canyon (it would be difficult to do all three). You will never regret it.

Enjoy your visit to America!

Myriam

P.S. I am planning to go to Australia for a month in 2012... any pointers?

<i>You have received many really great suggestions thus far. However, most of these are urban areas.</I>

Uhh...Negative.

No one "suggested" a cities itinerary.

The cities mentioned by responders are the exact same destination wishes clearly stated by the OP in the opening post:

<i>"Places we want to see include San Francisco, Las Vegas, New York and Washington..."</i>

Sorry most Americans are NOT rural. By far the majority live in cities. It's true there are a lot of empty areas (mountain ranges and state and national parks) - but that's not rural like - it's the great outdoors.

Agree that there are a lot of natural wonders to be seen in the US - if the OPs want to fit some of them in - but I cant honestly reco that they spend a lot of time looking at suburbs or miles of farmland versus SF, NYC or DC.

I think HVTraveler has a good point. There are some lovely rural or semirural areas to see close to many of the cities that the OP mentioned that might be a nice break from the hustle and bustle of those cities.

If you want to see the fall color in New England, you will need to do the itinerary sf3707 proposed in reverse, and I would suggest you do it south to north (mid-September to mid-October).

The unfortunate part about that is that the South can still be a bit hot and humid even in mid-September, and the Northeast will be cool (but not cold) in October, so you will need to pack clothing that you can layer to adjust to the different temperatures.

I'm a big fan of New York City and I would recommend to spend a couple of days in and not jsut see Manhattan.

Here is good likn that might be useful about NYC:
http://www.g8journey.com/experience/...p?featdestid=4

Have a great journey :-)

Thank you all for so much good feedback. It will take us some time to digest it and do the necessary homework and preparation but all comments are much appreciated.

Myriam, just a couple of comments about Australia. It is a big country and the best way to travel is by air - Qantas, Virgin, Tiger, Jetstar are the main options and all are reliable and safe. If travelling to our east coast then Melbourne, Sydney and the tropical north (Cairns) with the Great Barrier Reef are recommended. Regions around each of these are worth a visit. In the centre is Uluru and Perth in the west but these are a significant distance from the east. Perhaps you should post a request on Fodors too, but whatever you decide I hope you enjoy our country.

I have just returned from 2 weeks in the USA. We had a rushed week on the West Coast. We visited San Francisco (3 nights), Las Vegas (2 nights) and Los Angeles (2 nights).
Then we did NYC for 7 nights.
If I were to go again I would do NYC for 7 nights again. There is so much to see and do. I did not do all that I wanted to. And sometimes you just need some 'down time' without rushing around.
San Francisco - my husband and I both said that we should have stayed for 7 nights. Not only to see more of the city, but more of the surrounding areas. We really liked it a lot.
Las Vegas - won't go back. I was glad to see it. Especially the hotels, but it is not for me. If you like to gambol, or if you would like to go to the Grand Canyon then it is a good stop. Otherwise, forget it. Los Angeles was ok, however I wish that I could have explored areas such as Santa Barbara, Malibu and Pasadena.