The National Park of Teide, the island's volcano.
Teide is the tallest peak in Spain at a little over 12,000 ft. It is an active volcano an is uber impressive since the tiny island sits nearly at sea level and rises to this peak at the center of the island. All of the Canary Islands were formed volcanically, but it is most noticable here in Tenerife. There is a tram that takes you nearly to the top, but you have to request a permit to climb the peak (a short climb) info available on the website: http://reddeparquesnacionales.mma.es/parques/teide/guia_auto.htm#02