Seeing that El Teide is the second highest mountain in Europe, and the highest in Spain, how anyone from the UK could call it boring is beyond me, but we are all different I suppose.
Apart from that, the cost of the cable car is 22 Euros for adults and 11 Euros for kids under 14. Various companies run excursions to the cable car, but sorry, I don't know current prices. The trip to the cable car, which incidentally is at 2350 metres (7700 feet) above sea level, is worth it in itself just for the scenery, both on the way and when you get there. It is unique and stunning scenery.
The cable car takes you another 1200 metres higher, so then you are only 163 metres from the peak, however to go on foot to the top you NEED A PERMIT, which can only be obtained (in advance) from the National Park Office in Santa Cruz. However, on a good day, the views from the top of the cable car are spectacular and you've no need to climb the last 150 metres or so. You can check the weather in advance by phoning the cable car office (they speak English) on 922 010445, open between 9.00am and 4.00pm. It is true that it doesn't operate when there are strong winds or snow (yes it snows up there in the winter!)
I agree with other thread about La Gomera, its a tiring trip by coach, but well worth it if you take your own (rented) car, but of course you need to enjoy driving.
Any trip to the North of the Island is worth it 'cos everything except the climate is much better, i.e. scenery, food, culture, history, and architecture.
Tenerife certainly has more to offer than the concrete jungle (Los Gigantes being somewhat of an exception) which is the south of the island.