Well it really depends. I'd often relate it to something that the girl and I have see/shared together during the interaction before the phone number exchange (provided you got it IRL)
Otherwise, you can go for a little surprising/teasing. Say something that will want her to engage the convo further.
A few examples below (I don't like canned stuff but it's to give you the idea).
1)"Sarah, we don't tell our kids we met in a club. It's going to break them"
2)"You still owe me a drink after what happened last night Lea!" (make up some shit)
3) "SURPRISE!"
You get the idea...
I like to tell the girl's name in my texts as often as possible, I have read that your own name can trigger emotions when called and I noticed it worked on me as well.
You can also send her a funny picture/gif as an opener. A picture is worth thousand words they say
Good luck
Theberry