What a surprise to meet you here!
Imagine meeting you here! (cliché)
Fancy meeting you here! ( informal cliché)
Never thought I’d see you here!
What are you doing in this neck of the woods? neck of the woods=part of town
What are you doing in this part of town?
What are you doing out of the office? awkward
Where’ve you been hiding yourself?
What have you been up to?
Shouldn’t you be in school?
Shouldn’t you be at work?
Have you been keeping busy?
You been keeping busy? slang