There are hundreds of flights that arrive into Spain from all around Europe and beyond each day. Madrid has the country’s biggest airport. Note that some carriers say they fly into Barcelona but actually fly into Girona’s airport, which is about an hour and a half from Barcelona by train.
Renfe is Spain’s national carrier and it will get you around most of the country. If time is of the essence and you don’t mind spending a bit more money it’s worth considering Spain’s high-speed AVE trains which link Barcelona and Madrid in an incredible three hours. There are also AVE links between Madrid Cordoba and Seville.
It’s often cheaper to travel by bus than train and the range of services and companies mean you can pretty much get anywhere in Spain by bus. The best news of all is that they’re very often airconditioned and quite comfy.
If you’ve got an EU driver’s license you’ll have no problems hiring a car in Spain. Alternatively an International Drivers License, accompanied by your license by home, should be fine to rent a car.
You can get a boat or hydrofoil to any of the islands of Spain as well as to Morocco, which is a popular destination for many overlanders.