Check the car: Walk around the car and look inside, checking for damage. If you find any, ask the car rental company agent to mark it on your rental agreement. Take pictures so you can be confident about what the car was like to start with.
Check what fuel it takes: Open the cap to see if it's petrol or diesel. If you're not sure which fuel the car takes, ask the agent before you leave.
Look at the dashboard: Rental car fuel is usually provided on a 'full to full' basis, which means you start with a full tank, and you return the car with a full tank. Turn the key to check the tank is full (and the headlights work).
Sort your route: If your car has GPS, it's worth setting it to your language before you head off.
Take a moment: Once everyone's safely strapped in, take a minute to adjust the mirrors, move the seat, find reverse and check where the indicators are.
Look out for a petrol station: When you're heading off, look out for a petrol station. It'll save any last-minute stress about filling the car up before you take it back.