*Guten Tag* as a closing salutation is by far not as widely used as *Auf Wiedersehen*, which should be fine for all purposes where you [would also use *Sie* instead of *Du*](https://german.stackexchange.com/questions/77/how-can-a-native-english-speaker-know-when-it-is-appropriate-to-use-the-polite-s).

For a generic salutation that is more informal, you could use "**Tschüss!**", which I would translate as "See you!" or "Bye!". In southern Germany (especially Baden-Württemberg), there's also "**Ade!**" (emphasis on the *e*), and younger people often use "**Tschau!**".

*Tschüss*, *Ade* and *Tschau* have the same roots, [originally](http://de.wikipedia.org/wiki/Tsch%C3%BCs) even with a religious connotation (french *adieu*, "be with god" → *atschüs* → *tschüss*), but that is long gone.