Hi! John, your webmaster, here again. As I said on my "Money" page, I always change around £100 into euros before I go, then take a load of extra cash to change while I'm there. You'll find places to exchange money all over the place and they all have the exchange rates displayed outside. So, you can shop around a bit to find the best rate (although they only vary by a few centimos). Avoid such places as hotels and banks for this purpose.
So, my advice would be to change $100-200 into euros before you go and take a load more dollars to change in Tenerife, as you need to.