Clark is looking at some file and Lana stops by and
reminds him they were supposed to go for a cup
of coffee after work he has totally forgotten.
Lana: Well then I should probably leave you to it.

She's joking about leaving him to spend some time with the file?

Next round's on you. It's his treat next time?
Leave someone to it - leave them to get on with whatever they were doing
Next round's on you - you will buy the next drinks.
Thanks, opti!