Best time to visit
It depends on what you're planning. If you want to go to the beach, May to September are ideal (late summer can be particularly good!). Spring and fall are best for city trips, as it's not so hot then.
Driving
A national driving license is sufficient for EU citizens. The traffic signs and rules are by and large uniform within Europe.
The speed limits are 120 km/h on freeways, 100 km/h on dual carriageways, 90 km/h outside built-up areas and 50 km/h within built-up areas. The blood alcohol limit is 0.5 per mille.
You can find more information here.