Your hotel may be able to provide you with an adapter but probably not a converter; you could call or email them to inquire as some nicer hotels will offer this service. However, know that you will most likely need an adapter AND a converter since the electricity is different in the US and we have different shaped plugs/outlets from Europe.
There are some American appliances, such as hair dryers and electric razors, where you can switch the voltage from 110 to 220 and thus you do not need a converter. (Be sure to look before bringing it all the way to Spain!) However, even if you can switch the voltage, you will still need an adapter so that you can plug the appliance into the wall as American outlets are different from European outlets. Click
here for an earlier discussion on this topic.
Often hotels will already have hair dryers in the room so you may not need to bring one at all. Check the hotel's web site or contact them directly so that you don't have to worry about carrying one more thing from home and you have more room for souvenirs.
Hope this helps!