Make sure to bring comfortable shoes with you, as you can walk pretty much all around the circuit and half the grandstands are accessible. In 2013 one could buy tickets online but also at the gates 3 and 7 at the track. Don't miss the outside area of turn 3, not many people discovered that you could walk to this spot.
http://www.circuitcat.com/en/f1/tarifas.php
If you scroll down, you'll see the map of the circuit with the gates and all the grandstands. Below K is a tunnel to the inside of T6. There were also guided tours in 2013 that took you around the track and to the inside of T12 for 50€, while the normal entry was 20€, if i remember correctly.
Dear FIA, if you read this, please pm me for a redesign of the Technical Regulations to avoid finger nose shapes for 2016! :-)