1. I haven't seen you for a long time. / Long time no see.
2. How have you been?
3. How's everything going?
4. What's happening?
5. What have you been up to?
6. It's really nice to see you again.
7. What keeps you so busy lately?
8. What's up?
9. What's going on?
10. It's been a long time. How are things treating you?