Last September we went to Callao Salvaje on the west coast; I took the mtb and got a couple of rides in, it was brilliant, so definitely take it, you’ll be kicking yourself if you don’t.
Get a lift or the bus up to Las Lajas, enjoy the plummet down from 2300m to sea level or thereabouts. There are quite a few tracks just there; have a look on openstreetmap, the strava heatmap or if you’re stuck I can try to find the gpx tracks from my two rides.
There was one trail in particular, through the viewpoint at Boca del Paso above Adeje, which was both brilliant and possibly one of the longest, really-hard-but-just-about-rideable descents I’ve ever done. I’d love to go back, Tenerife has a bad rep regarding British tourists but it’s an amazing place.