ATM? Exchange booth? Airport? Credit Cards?

ATM and Credit Cards are best as you will get the wholesale exchange rate.

The only things to keep in mind are any ATM fees your bank might charge for a foreign withdrawal and your credit card's foreign exchange fee (which varies from 1% to 3%). Even with these fees you should come out ahead of any foreign exchange booth. (And there are ATM's in the airports so you don't have to worry about not having any euros in order to get where you're headed.)

I have found that credit cards and ATM withdrawals give the best exchange rates.

In the UK, some ATM machines had no fee - others about $1.50 or so per withdrawal (not sure about Spain) -- so take out a lot at a time.

Some credit cards have a surcharge for overseas use -- my Chase card did, but but my MBNA card did not, so I used it excludively in Europe (but took the other card as an emergency backup).

I agree that you should let the card issuers know about your travel plans in advance - they put a notation in your record, so they don't view the overseas charges with suspicion.

Also, write down your card numbers and the emergency phone numbers for each card in case you lose them.