Don't do this - do it yourself. You'll have WAY more fun.
Skip the beach - the water off California is cold even in summer (and yes, July is summer). Otherwise, yep, that all sounds good.
You'll want to pull up Google Maps and look at where you want to go. Disneyland and Six Flags are both generally in Los Angeles. Yosemite is further up north, about halfway between SF and Las Vegas. The Grand Canyon is over on the far side of Las Vegas. That's a lot of ground to cover.
I'd actually give Yosemite the flick, and look at more national parks over near the Grand Canyon. The great thing about visiting in July is that you can
do a loop a bit like the one on this map, starting in Vegas and hitting the Valley of Fire, Zion, Bryce Canyon, Antelope Canyon, and the Grand Canyon all in about four or five days. I've done this loop three times now - it's brilliant driving, the scenery is unmissable, and it's something very very unique. It's a classic American road trip.
And if your boyfriend likes to drive, get him a treat. Hertz in Las Vegas will rent you a
Chevy Corvette Stingray for dirt-cheap - it's the ultimate American muscle car, and it's ABSOLUTELY BRILLIANT for doing great American road trips. You need to drop it back to where you rented it from - they won't let you do a one-way rental - so you'll want to start and end in Vegas, but it's a fantastic way to travel. (It's got a surprising amount of luggage space for a Ferrari-smashing two-seater sports car too - you'll fit two large suitcases in the back no worries.)
That's going to be VERY tight. It's a six-hour cross-country flight, plus a day or so to get up to Niagara, and a day to get back, and then another day blown flying. I'd rule out Niagara. (And frankly the shopping in NYC is exactly the same as in LA, SF, or LV.
Stick to the west coast and you'll be fine - tight but fine. You're not going to be able to get over to the east coast in two weeks and still have time to enjoy it.
I'm not sure you've got a handle on how big America is. Driving Disneyland to Yosemite is five solid hours - six or more if you hit traffic through Los Angeles. Los Angeles to Las Vegas is four hours nonstop at 130km/h, and Vegas to the Grand Canyon is four hours again. You're going to be doing a LOT of driving.
Nope, GPS works brilliantly. You'll never get lost. Plus, the interstate highways are easy to navigate.
Call it $1500 per for flights, $200/night for hotels (that's pretty generous - it'll average out between the expensive stuff in the cities and the cheap stuff on the road), $100/day for meals, and bung in some extra for miscellaneous expenses.