We go for a week every year and each time say, "well, maybe we should spend a day off site." Never happens. Enjoy this first trip. Wait until your second trip to consider a day off site.
We have never rented a car, so I can't talk about the benefits of driving. But even when we had strollers and extra bags, the bus seemed so much easier. It was one less expense, one less hour at the airport coming and going, one less thing to worry about.
Consider bringing a small cooler (six-pack size) We make sandwiches and bring water bottles. Food can get pricey and take time out of the day waiting for it. We tend to eat on the run, when and where ever it suits us. One suggestion I heard was to buy the juice boxes and freeze them the night before to serve as ice packs in the cooler.
Whatever you do, don't get overwhelmed with the size, options, crowds, etc. Yes, park tix are pricey so you feel you need to do as many rides as possible. But a big part of that daily entry fee is the ambiance, the parades, the shows, fireworks, characters, shopping, etc. Like most folks, you'll get back there someday to do the things you couldn't do this time.