Saying you “had a nice time”
Well, if you had at least a pleasant time, say that. And thank him. But, if you didn’t like the guy, don’t let him live with the delusion that you do for the night. Don’t let him go home and dream about you all night. Don’t get his hopes up. Tell him right there and then–as uncomfortable as it may be–that you don’t see it going anywhere, and that you didn’t want to waste his time.