Hi Aussie Air,
Not to sound like an elitist pain in the ass but personally I'd avoid every place you mentioned in your original query (at the start of this thread) UNLESS you are CERTAIN that you really LOVE crowded, overpriced, tourist-choked theme parks and attractions. Besides, from my finite experience one theme park looks pretty much like another (I've been to all but one of the places on your list, though thankfully not during the past 10 years).
By all means go see NYC if you haven't already done so. Personally, I'd avoid the tourist hot spots (always anticlimactic) and simply walk around (be sure to take proper precautions as mentioned above... good advice when travelling ANYWHERE). I lived in NYC for 25 years and take my word for it, it's best viewed from street level, on foot in decent weather (late spring or fall is ideal).
If you make it to LA, I'd strongly suggest a visit to the Getty Center if you're any kind of art and/or museum and/or architecture fan (and even if you're not). It cost 1.3 billion dollars to build and looks as if it's really worth every penny.
Forests? Well, America is somwhat vast and I've had great hikes in the temperate rain forests in the pacific northwest (Oregon and Washington) as well as many other places (South Dakota comes to mind). Parts of Arizona and Montana are also stunning when it comes to natural beauty as well. However, I'd imagine that Australia is pretty good in that area itself (from what I've seen on TV, films, etc... I had a chance to go years ago but didn't jump on the opportunity) so if your time in the US is limited I wouldn't go out of your way to SD or someplace just for a walk unless you're really into that sort of thing.
Good luck and enjoy your trip.