Formal:
* Até breve! (This is the most common and versatile option)
* Até já! (This is a bit more informal than "Até breve!")
Informal:
* Falo contigo mais tarde! (Literally: "I'll talk to you later!")
* A gente se fala! (Literally: "We'll talk!")
* Até logo! (This is a bit more casual than "Até breve!")
Other options:
* Mande um abraço! (Send my regards!)
* Até mais! (See you later!)
* Falamos depois! (We'll talk later!)
The best option to use will depend on the relationship you have with the person you're speaking to and the situation.