Is price or convenience more important to you?
if price, just go for the cheapest, but you might have to get a shuttle bus to the car lot. If convenience, then look to see if there are any rental companies that are on site..those you can just walk to.
you will get an automatic car,
google about toll road transponders and work out if you want to use the rental car transponder (they might charge a fee) or if its possible to pay as you go..most tollroads make this quite difficult now. If you are using the rental car transponder, open it as soon as you get in the car and leave it open for the whole trip.
look at the insurance options ahead of time and see if any of your credit cards provide rental car insurance top ups. If they do, book the car with that card
once you have picked the company you want to use, see if you can sign up for the loyalty scheme and if that means you can just grab and go when you arrive. It saves loads of time waiting in queues once you arrive.
renting childrens car seats is usually a rip off, you will want gps driving assistance/maps as the US roads can be very confusing if you arent used to them. If your phone wont give you enough data in the US, consider renting the unit with the car