yeah, agree that Santa Monica is the place to stay in L.A.......don't stay in Hollywood/west hollywood, it really is a dump of a place. If you can, hire a car and drive south to Newport Beach, Huntington Beach, Manhatten Beach and Redondo beach. Going the other way, Santa Barbara and Malibu are worth a drive too.

San Francisco..........personally i thought it was a hole, very disappointed by my San Francisco experience. Other than shopping and Golden Gate bridge i found the city to be quite sterile and boring. If i was you, i would spend less time in San Francisco and use that time to explore the Big Sur coastline, Napa Valley and possibly even Yosemite national park. Drive from L.A to San Fran via Big Sur or vice versa if you are landing in San Fran first.

New York i stayed in Chelsea which was a cool place to stay and more affordable than most other locations in Manhatten. It's also a very short distance to Greenwich Village and meatpacking district both of which have lot's of cool bars, restaurants and clubs. If you want to do something a bit left field, i hired a bike and rode from Central Park all the way down the Hudson River bike path over the Brooklyn bridge and into Brooklyn. I then returned the bike to Battery Park. You get a totally different perspective on Manhatten when you do the ride. It's about a 4 hour ride depending on how often you stop etc but it's relaxed and easy and a great way to see parts of the city you otherwise wouldn't see.