• VISAS:
  Generally, travel in western and central Europe does not require that you carry a visa if you'll be there for less than 3 months, but if there's any chance you'll be visiting any eastern European countries (even for less than 30 days), you may need one. The sites listed above can help you figure out if you need one. NOTE: This information is with consideration as an American passport holder.
• DRIVING:
  If you want to drive, you'll need an International Driving Permit (aka IDP). Many countries allow driving for a short period (given you have your license already), but the laws change from country to country. To be safe, you can pick up an an IDP at any AAA office for $10.
  
  For renting cars in Europe, here are some requirements (varies according to country): must be at least 21 years old and have a drivers license (more specifically an IDP). Also be forewarned that cars with automatic transmission aren't as readily available.
• STUDENT ID:
  The International Student Identity Card (aka ISIC) is the most widely accepted student identity card. It can get you discounts on travel, hotels, museums and historical sites. These cards are $20. Ask for the ISIC Handbook to find where discounts are available.
  
• MONEY:
  You get a better rate if you exchange your currency overseas rather than in the US. Best rates are available when you use a local ATM. The worst rates are given in convenience areas like airports or train stations. Credit cards are also a great way to use money (Your best bets are MasterCard and Visa for acceptance). Travelers Cheques are always a safe bet.

Before leaving make sure you have copies of all of the above information. Put them in a different location from originals in case you are separated from them.