Book your hire car in advance. Leave it until you show up at the car-hire desk and you'll get taken for a ride. You might also find yourself facing a restricted choice, and possibly shoehorned into a "prestige" vehicle and paying a hefty surcharge for the privilege of a leather-bound steering wheel.
If you're driving in unfamiliar places you need a GPS, but hiring one from your car-hire operator will probably be expensive. You might be better off purchasing an in-car GPS at your destination, or upgrade your own device with the relevant maps. Another option is a GPS system for your smartphone or tablet. I use Sygic GPS maps loaded onto an iPad, which give turn-by-turn voice commands and speed camera warnings with no data connection required, and the price is reasonable.
While you will never be asked to show an International Driving Permit (IDP) when you hire a car, it might be a different story if you run foul of the law. The IDP is a translation of your licence into other major world languages so that a police officer in Brazil or Turkey can check its validity. While you might not think that would apply in an English-speaking country, Texas is one place where an Australian-licenced driver without an IDP can be fined for driving without a licence. Your state motoring organisation can fit you up with an IDP, for a fee under $A40.
You need insurance to cover you for damage to persons or property caused by a motor vehicle that you are driving.
All hire cars in Europe come with Collision Damage Waiver. If you have an accident and damage the vehicle, your liability will be limited to the excess, which might be anything from about €200 to €2000 ($A277 to $A2770), depending on the country and the car rental company. You can reduce this excess at the time you hire the vehicle and the desk crew will try and push you in this direction, since this is a moneyspinner for car-hire operators. Chances are they will also try and upsell you with insurance to cover damage to tyres and windscreen.
When you hire a car in the US you need cover for Collision Damage Waiver (CDW) and Loss Damage Waiver (LDW), since you may be liable for the total replacement cost of the vehicle.
All the motor vehicle insurance cover you need may be included in your travel insurance policy. Some credit cards will provide insurance cover for your rental vehicle when you charge the hire to your card. Check with your card provider to see if this applies.
In Europe especially, diesel wins hands down over petrol. Diesel is cheaper, diesel fuel consumption is lower than with a petrol engine and there really is no downside. European car makers now have many years of experience building even small cars with diesel engines and they're smooth, responsive and easy to drive.