Here are some other options:
* "Cymryd gofal!" (Take care!)
* "Hyd yn hyn!" (Until then!)
* "Ffarwel!" (Farewell!)
* "Dyma fi'n mynd!" (I'm going now!)
The choice depends on the context and the level of formality. "Hwyl fawr!" is the most versatile and can be used in almost any situation.