Casual:
* Bis später! (Most common and versatile)
* Bis dann! (Similar to "Bis später!")
* Ciao! (Borrowed from Italian, very common)
* Mach's gut! (Literally "Do it well", implying have a good time)
More informal (and possibly a bit cheeky):
* Bis bald! (See you soon, implies you'll see the person again soon)
* Bis denne! (A more informal version of "Bis dann")
* Tschau! (Short for "Tschüss", another slang for "goodbye")
* Schönen Abend noch! (Have a nice evening, suitable for the evening)
* Bis denne, dann! (A combination of "Bis denne" and "Bis dann", very informal)
Remember: The best way to choose the right expression depends on the context and your relationship with the person. If you're unsure, "Bis später!" is always a safe bet.