Two 18 year old girls traveling alone...suggestions on where to go?

Summer here is winter in New Zealand and Australia, so I would skip those.

Ireland would be beautiful that time of year, has youth hostels and the coast is awesome - cool castles. England is interesting from an educational stand-point and the beaches of France could be a blast.

Alaska, nah...

In terms of being "safe", European countries have really low crime rates outside the bad parts of the major cities. Same common sense you would use here like don't walk down the dark alley at 2:00am kind of thing.

If you have the money, I would do Europe - check into Eurorail passes before you go and you can go all over the place for one price.

If you want to sell your parents on it, look up the violent crime rates in the countries you want to visit compared to the United States. The worst city in the major European countries probably has lower violent crime rates than any city in California.

Id hate to sound like im ruining your dream but this is extremely dangerous. Men look for scenarios like this at least travel in a larger group of females. Two young teenage girls are an easy target.

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I live in Australia, if you are into beaches in summer, parties and sun etc. then as one of the others said skip here, well at least the south bit. Otherwise Australia is an option there's plenty to do.
I also want to travel in "summer" 2010 (ie. december2010/january 2011) after school is over but kind of thinking NYC and San Francisco (your winter, which is actually good because the weather here kind of sucks in summer too, its only spring and autumn that have anything going for them) maybe, if I can get the money together and my mother will not cry because she thinks I am going to die.
If you want to do something really awesome perhaps join a company tour to somewhere a bit far out (or not)? I have no idea if you want that kind of outing but I went to Malaysia last year and am going to Nepal end of this year with an Aussie company, its a bit more expensive than going your own way but its safer and if you get a good company its a real advantage.

Sounds fantastic, and definitely something I'll encourage my kids to do when they're older. My 2 friends and I spent 3mths back-packing around Europe on a rail card when I was 18, a fab experience. I appreciate you'll be worried, but you can't watch over your adult children forever and this will be a great opportunity for your daughter to test her independence, see some amazing sights and experience different cultures. If you ask her to check in regularly by email and Skype that will help put your mind at rest a little while she's gone, as will having the details of her itinerary.

Asia - thailand, japan, korea, indonesia, singapore, hong kong, philippines...

South America - brazil, ecuador, peru, argentina...

Pacific Ocean - Guam, Fiji, Papua New Guinea

OR

The Carribeans! - Cuba, Dominican Republic, Barbados, Bahamas, Haiti, Trinidad & Tobago!

Honestly going out of the country is prob not that safe, you should take a trip close to home, but not california, such as. the grand canyon, or Yosemite. however fun, and sometimes more cheap when traveling from cali, hawaii is the best choice :)

Backpacking or no.

I would not go to Alaska or Canada for my graduation trip.
I would choose to backpack Europe.
Out of the list you have put out, I would choose Greece because there is a coast, and you can easily travel around using trains.

Staying at hostels is a must because it saves money and you meet a lot of cool poeple there. They speak english, too.

Personally, I went backpacking with my brother last summer as my graduation trip.
We flew from beijing, to xi'an, to shanghai. And it was AMAZING. Hostels were great there. They spoke good english at the hostels.
We were the only americans at the hostel lol.
China may be a little of a culture shock for you cali girls, but thats what makes it a trip of a life time.

If you want more info about my trip, just send me a message.

It depends on what you like to do or see when you travel. If you are outdoors type then you should go to Alaska or maybe come to Arizona and see the Grand Canyon and a side trip to Lake Powell. There are a few different raft trips you can take down the Colorado River. You could have a lot of fun.

try britain.
its where i am from and summer 2010 and i'm going back once i graduate HS this year too.