El Teide hike is one of the most interesting I have been to, the volcanic scenery is wonderful, colourful and dramatic. The highest mountain in Spain, 3718m high can be visited as one of the many hiking trails on Tenerife. You can take easy walks and tough hikes, from the route around the rocks to the climb of El Teide itself.
Another possibility to get to the top is to take the funicular (50 Euro) that takes you to the height of cca. 3,500m.You need a special permit for the last 200m that you only get in Santa Cruz de Tenerife, but in my opinion, you don't need to go up to the peak.
It's very hot and dry up there and there are only 2 places to get drinks so you need to take plenty of water with you. It's absolutely not for faint-hearted as the air is very thin and smallest physical activity makes the breathing difficult.
It is about one hour drive from Playa de las Americas, along a main road which winds its way up through the hills and forests, about an hour's drive, you can also fetch a bus from Puerto de la Cruz which goes twice a day.