Greetings, Iron Wing!
I have found that from the time I step off the plane in Frankfurt until the end of EuroFURence - about 1 week - that 300 Euros will handle all of my Non-EF spending, for things like local transport, off-site meals, local-bought toiletries, tours and admissions, that sort of thing.
You may have your own source for Euros - My bank can provide me with the cash if I let them know 2 weeks ahead of time I need it, or from American Automobile Association ("Triple-A", or AAA) through their service offices, again with some advance notice - but I've found the best way to handle it (and get the best exchange rate) is to use an ATM machine (a "Geldautomat" in German) at the airport; Frankfurt airport has several, and the new Berlin Int'l ought to have plenty, too.
I recommend bringing both an ATM/Debit card AND a credit card with you; BUY things with the credit card, to avoid hitting a daily spending limit on your bank account, and GET CASH with the ATM/Debit card, to avoid onerous interest charges which Credit Card companies impose on "Cash Advances" (To the Credit Card company, getting cash is a LOAN, and interest at 15% - or worse - is charged, starting from the moment of the "advance"...and it's not forgiven under the "grace" period like your monthly purchases are, you can't dodge it).
******WARNING******
At least a week before your trip, advise both your bank AND the Credit Card company of the dates and destinations of your trip; otherwise, you could show up in Germany and have your cards refused, as the respective card issuers will immediately suspect fraudulent use...and calling the "toll-free" customer service numbers from a German pay phone is nigh impossible. Alerting them via phone or on-line, like the day before your trip, or even 2 days before, is not enough lead time. Believe me, I've been caught by this TWICE. Thank God for well-connected concierges and the kindness of European Furry Friends!
You might also take the opportunity to see what your daily withdrawal limit is on your ATM/debit card, and perhaps have it raised.
******END OF WARNING******
As for your EF-related spending...If you are staying at the convention hotel, or at the recommended alternate (Andel's Vienna House), you can charge your meals to your hotel room (I have had to stay at Andel's the last three years, but have lunch and dinner at the Estrel, so I spend cash there, or charge to my credit card).
Inside the actual convention space, EVERYTHING is on a cash basis, including what is sold in the Dealers' Den, and at the Art Show, and at the Charity Auction. What you will need for Art and Merchandise is entirely your call, but there is a Geldautomat AT THE HOTEL. It has been known to "run out" over the span of the convention, and some years arrangements had been made for trailer-based MOBILE ATMs to be set up on-site, but I wouldn't count on that. I am told that there are ATM's about a block away towards the McDonald's down Sonnenallee, and there are ATMs in the opposite direction towards the center of Neukoln.
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Oh...and you will need a new wallet.
Seriously - you will want a wallet with an attached coin-pouch, or zippered compartment, or some way to carry coins. This is because of the way a Euro is divided into cents.
The Euro has 100 cents. The coins (and bills) are denominated in a 1:2:5 manner, like this: 1 cent, 2 cents, 5 cents (these are copper coins), 10 cents, 20 cents, 50 cents (these are brass-toned coins), 1 Euro, 2 Euros (these are bi-metal coins, like the Canadian Dollar, or "loonie" coin).
...That's right - the Euro itself is a coin, as well as the next denomination up, the 2-Euro coin. The first bill is 5 Euros, then 10, 20, 50, 100, 200, and 500 euros.
I think that most Americans don't trifle with coins much; this is why every attempt to replace the 1-Dollar bill with some sort of coin never catches on. We tend to pay with bills, receive change, and empty it out of our pockets at day's end, on our dresser or in a coin bank or something.
In Europe, you will be expected to pay for items under 5 Euros in cost with those coins. Waiters in restaurants and cashiers in stores will of course make change, but you are gonna be burdened down with a LOT of coins otherwise.
Likewise, at the opposite end of the spectrum, if you use a Geldautomat which offers you a choice of denominations for your withdrawal, unless you plan on dropping 1,000 or so at the Art show or for the Charity, avoid the high-value notes; 50 Euro bills would be the largest single bill you should need, and for many places of business, paying for a simple meal or sundry purchase with one would be "pushing it", and you wouldn't be making any friends in the process.
Back to wallets - a "Bikers'" or "Truckers'" wallet, the flat black leather kind with a small chain, will hold both your travelling Dollars, your Euro bills, and they have a zippered compartment for the coins. Gift shops at the airport (or even the hotel) may have wallets for sale, and they will have a coin pouch. I even bought a handy tri-fold at the general store in the shopping complex across from the McDonald's down Sonnenallee, for about 6 Euros, which holds a couple credit cards, Euro bills, Visitor's Travel card and room key card, and has a coin pouch on the outside. Whole thing about the size of a pack of cigarettes.
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Well, that's a LOT more information than you asked for, but I hope that it might be useful information for other American "first timers" making the trip to Berlin for EF this year. I hope some of it was useful to you!
Good Luck!
