Formal:
* Good morning/afternoon/evening.
* How are you?
* It's nice to meet you.
* Thank you for meeting with me.
Informal:
* How's it going?
* What's up?
* How are you doing?
* Long time no see!
Other options:
* I'm [your name]. (To introduce yourself)
* Are you enjoying the [event/weather/etc.]?
* I'm here for the [meeting/appointment/etc.].
* I'm looking forward to [talking/learning about/etc.].
Remember: The best thing to say after "hello" depends on the situation and your relationship with the person. Be polite and friendly, and try to tailor your response to the conversation!