You would like to travel for a whole day during the week?
In that case one of our Länder-Ticket is your best choice. You can use
the ticket with up to five people and you can book the ticket online.

For up to 5 persons traveling together or for parents (or one parent or
grandparent) traveling with any number of their children or
grandchildren under 15. The ticket is only valid for one day in 2nd class.

It can be used on all local trains (Interregio-Express,
Regional-Express, Regionalbahn, S-Bahn) and the entire transport system
of the Verkehrsverbünde. In some Bundesländer you can buy a supplement
of 13 EUR to be able to take IR- or D-trains. Tariffs, terms and
conditons can vary.

Taken From
http://www.bahn.de/international/view/en/prices/germany/laender_tickets.shtml

Travel
in the Bundesland (federal state) of your choice. Most Länder-Tickets
are valid also on Saturday and Sunday. They are valid from Monday to
Friday for one day of your choice from 9 a.m. until 3 a.m. of the
following day (on bank holidays for the whole day). Some tickets can
also be used for nighttime travel, they are valid from 7 p.m. until 6
a.m. the next morning.