I like the night out idea. Start with a good bottle of champagne, gift cards to their fav restaurants and theater, and ice cream place. They will need a little stress busting after the wedding, and these things they can use all on one night, or stretch it out.
Go find a special keepsake, go look at Things remembered at the mall, and see if there is something suitable that can be engraved.
A nice frame with their fav wedding picture is traditional, so is a photo albumn of your day. But do these things in addition to something personal for them.
Do a collage of things from the wedding, suitable for framing.
Go to the jewelers, and see what they have, a nice watch, engraved, for Dad, a piece of jewelry for Mom.
Do the parents have a fav weekend getaway spot? tell them to make plans, and pay for the room and give them money for food.