With the Quer durch´s Land-Ticket (link in German language only) you can travel through whole Germany and parts of neighbor-regions
It´s for 44€ – and up to 4 fellow passengers can travel with you for 8€ per person. Don´t forget to note the name of all passengers before you start your travel.
Your own kids and grandchilds to the age of 15 are free!
Children till the age of 6 are always free in the Deutsche Bahn
Sure, you can get one way tickets for a singel trip, but if you want to return on the same day, it´s worth tho check out the „Tageskarte“. You can also give it to a friend, when you´ve finished your trip.
If you are traveling together with up to 5 persons, you can get a „Gruppenkarte“. So you can save a lot of money. For a group-ticket for up to 8 persons ask the information