Informal and friendly:
* "Nada mucho, ¿y tú?" (Not much, what about you?)
* "Todo bien, ¿y tú?" (Everything's good, what about you?)
* "Tranquilo, ¿y tú?" (Chillin', what about you?)
* "¿Qué onda?" (What's up?)
* "Pura vida!" (Life is good! - Costa Rican slang)
* "No mucho, ¿qué pasó?" (Not much, what happened?)
More formal:
* "Bien, gracias. ¿Y tú?" (Good, thank you. And you?)
* "Estoy bien, ¿y usted?" (I'm fine, and you?)
Humorous:
* "Esperando que llegue el fin de semana" (Waiting for the weekend to arrive)
* "Tratando de sobrevivir" (Trying to survive)
If you don't know Spanish:
* You could simply smile and say "Hello!" or "Good to see you."
* If you want to be playful, you could respond with "Que paso?" and see if they get the joke.
Remember to adjust your response based on your relationship with the person and the situation.