Hi there,
I checked out Eurail and RailEurope and they seem to offer mostly 5 days and upwards passes. As you are after a one day pass, you can purchase this when you arrive to Berlin.
In terms of
attractions, Berlin has many popular museums as well as the popular Potsdamer Platz Arcades where you can do plenty of shopping.
The Berlin Wall is another top attraction although there isn"t terribly much left of the structure but the Brandenburg Gate still stands. If you want a better understanding of the history behind the wall, you should visit the
Berlin Wall Museum in Berlin.
Berlin sightseeing tours are available, although I would suggest exploring Berlin on your own as buses run throughout the city and are certainly a cheaper option than a guided tour.
Hope this helps
Vacationbuddy