I think that if you visit Spain, you'll have a whale of a time and would definitely have no regrets. It is a cheap place (you wouldn't have to spend a lot of money once you're there), but the service and value for money that you'd get are excellent.
Above all, the people in Spain are among the nicest anywhere in the world.
You can also travel from one place to another by bus, which again are very cheap, yet very comfortable.
Hotels are cheaper than elsewhere in Europe and the quality is second to none.
Barcelona and Madrid are must.
I was in Spain for the first time in January of this year - I liked it so much that I have since returned twice.
Regarding PASSPORTS: it depends on which country issued your passport. As far as I know, people holding US passports can stay up to 3 months in Spain (and the other EU countries signatories of the Schengen Treaty) without the need of a Visa. But it is always best and safest to phone up the Embassy or the Consulate (in your home country) of the countries you intend to visit.
Enjoy your trip.