I visited Tenerife in January 2011. I have great memmories on it, because it was a special trip when me and my friend crossed a half of the island by walk overnighting in the wild nature. I am pretty sure that it was not completely legal in National Park. I am also sure that it will never happen to me again, because I am getting older prefering hotels and not prefering heavy backpacks anymore... So, it was a unique experience in all aspects.
We walked from Vilaflor located near the southern edge of Teide caldera. It was for the first time I was in the vulcanic landscape, and I cannot describe properly how I felt when have been climbing Guajara mountain, seeing Teide mountain and the entire caldera for the first time. We refused using "teleferico" to reach the summit of Teide and walked towards Refugio de Altavista, where we have pre-booked an accommodation by phone. We could not climbed the peak because of ice plates that might be dangerous. Then, we walked to the east to Cumbre Dorsal mountain range.
What I loved was all the contrasts: blue sky vs. black basalt rocks such as Huevos del Teide vs. ochre or red soil; sunny days vs. freeze of night; Moon-like landscape vs. green pine forests outside the caldera vs. blue ocean. Furthermore, the color shades of rocks within the caldera changed during the day.
To conclude: Teide National Park embodies one of the most beautiful landscape I have ever seen.