If you're hiring a car get one where you take it back with a full tank, we drove all week and never even got it half empty
If doing Teide apply for a permit now although you may be to late? You could always get the last cable cable car up and wait for the guard to go down, nip up to the summit and you'd have to walk down though.
Masca gorge is highly recommended as well
Forget that! Wrong Island