Welcome to Flyertalk, xicanaloa

I'll try to answer some of your questions...

Flying to another city and taking the train? Hmmm... you'll be tired after your long overnight flight, and if you go on a train journey after your restless night it's likely you'll just fall asleep and not see anything. Probably best to leave the exploring for some other time (I presume they won't make you work seven days a week?). Also, FRA is the main international hub for Germany, some of the other ones don't even have even have flight to North America, so flying into FRA sort of makes sense.

Telephones - definitely go for a local prepaid, and use Skype/Skype Out for calls to the USA.

Money - you don't really want to carry large wads of cash across the Atlantic, so you will need to find some way of withdrawing money while you are in Germany. Do you have an ATM card? Germany is very much a "cash country", so using cash to pay for everything should not be a problem, except in that not having a credit or debit card makes it difficult to prebook things (such as cheap train tickets)

As for places to see... there are so many to choose from, so it's difficult to recommend somewhere. What are your interests, what sort of things would you like to see and do?