I have usually found flights via Kayak or Google Flights, and then booked via preferred method (whether direct, another site, etc). I use Booking.com for accommodation. I like Alamo for car rental (though I bought a bus and kept it in the US for the last five years to save spending on car rental each trip). If you're flying between cities and don't need to head out far, you'll avoid hassle skipping car rental and dodging park, etc.
When I go over, I just take my debit card and a backup card. My wife usually withdraws cash and deals with tipping (I can't stand it) and cash purchases like campgrounds; keep a variety of denominations between you. If the AU credit card is ever an issue, it's really only been when pre-paying at the pump for fuel, and then you just go inside to pre-pay and they refund any not used. If it asks for a zipcode, you can sometimes prefix with a 0 to get around the five-digit check. My provider refunds fees for international transactions. If you have 3-4 cards between the two of you, with a bit of bank or type variety, you should be fine.
For phone, if it's a shorter trip, I pay per day for roaming to extend my AU plan abroad. This is usually $5-10/day, so gets expensive on multi-month trips (did that once - oops). Otherwise I use Airalo to get a data-only eSIM and set it up so that the US SIM handles data and the existing AU SIM receives my calls. You can get SIMs that give you a US number as well if you plan to be making a fair number of local calls. You could get one data-only eSIM and the other person just hotspots off it.
In LA: The Getty, The Apple Pan (classic burger joint), Big Dean's in Santa Monica (another burger place) and then walk out on the pier, Griffith Observatory, walk from Santa Monica to Venice Beach, stroll Venice Canals, Baja Cantina in Marina Del Rey for Mexican in the evening. I've always stayed on the western side (Santa Monica, Venice, Beverly Hills, West Hollywood all decent) so not sure about Downtown.
In LV: stay on the Strip and be prepared to do a lot of walking as the casinos are massive. Worth seeing both the themed casinos (Paris, Venetian, Bellagio, etc) and the premium ones (Wynn, Encore). Stayed at Encore years ago, but not sure what that's like these days. Places like Paris and Venetian are so ridiculous they're great, IMO. Some of the questionable attire on the Strip is unique.
In NY: all the spots are pretty obvious here - Central Park, The High Line, other smaller parks, the big galleries (Guggenheim, Met, MoMA, Whitney), Nat History Museum, wander out on the Brooklyn Bridge, Brooklyn Brewery.