@Karen4052 Just tell them verbally or though messaging that you'll offer them X% discount if they want to come back and stay again. Messaging is better because you then have a written reminder to jog your memory. You can also put a note in their reservation.
Ask them to remind you of the discount you offered when they book again, and you can send them a special offer with the discount included. Better yet, tell them to request to book and remind you of the offer. Then you can send a special offer.