“Come on! What do you say?”

You meet a girl who you'd like to go out with at a dance club. You ask her on a date, but she doesn't answer. You want to encourage her to answer. You say:

Come on! What do you say?

Come on!

You say "come on" to someone when you're trying to convince them to do something. It's used in casual situations. "Come on" means something like "Don't worry about it; just do it." Here is another example:

Come on, stay for one more round.

What do you say?

Ask this question when you've made an offer to someone, but they're hesitant to take it. In the example above, the speaker offered to take the girl out on a date, but she was hesitant to say "yes" to going out with him.

Use "What do you say?" after you've made your offer, but before the person you're talking to has given their answer. If they've already said "no", it doesn't make sense to ask "What do you say?"