Spain is dotted throughout the country with international airports and it's connections with the rest of Europe and the world are excellent. Most of this country is also well covered by both bus and rail networks. On the other hand, if your trip to Spain is part of a wider European tour, then it may be worth investing in a rail pass, such as the InterRail ticket. These can be purchased from Rail Stations, Internet, etc. The RENFE web site is http://www.renfe.com/ and is available in a number of languages.
Spain has an ever growing network of high-speed trains (AVE), and mostly at very reasonable prices compared with most other countries. Seats on the AVE can also be booked at Rail Stations, Travel Agencies and on the Internet.
If your distance of travel is not very great, buses may be quicker than a train and will also normally take you closer to your destination. This is mainly because the majority of train stations are not always near the centre of the city they serve, and you have no guarantee of a connecting bus. However, things are improving very quickly, with many Bus Stations being planned next to Railway Stations. An example of this is in Malaga which has Bus, Train and Metro (currently being built) stations next to each other, making transfers very easy. Barcelona's new station - currently being planned / built will be another example.
Hiring a car in Spain will never be your cheapest option, but it might be the most convenient. If you want to explore some of the Spain's harder-to-reach regions, like Alpujarras, the Rias Bajas (to the west of Santiago de Compostela) or the region around Cadiz and Tarifa, renting a car is your best choice. Hire Car prices vary enormously, but generally, if you can hire a car near an airport where there is a lot of competition, prices are much more reasonably. The further inland you go, the more difficult it is to find a hire car (except in the cities) and the more expensive it becomes.
Taxis are a good way to get around all Spain cities and towns, but they can get rather expensive. Taxi stands are numerous, and you can easily hail a taxi from the street. Always ask for the price first.
If you are holidaying in Spain and none of the above options appeal or are unsuitable, your hotel reception, holiday company courier or a local travel agent may be able to suggest various coach tours or routes. These tours tend to be based mostly in seaside resorts and will often offer a different destination each day of the week.
Some of the timeshare resorts will also provide free shuttles to and from the airport at the most popular Spanish destinations.
As a last resort, hire a donkey ! You might not get very far, but you will see more of the real Spain than you would with faster modes of transport.