I had that exact thing happen to me with my granddaughter.....I first sent a card with an explanation....Then when it was done, I sent the quilt. Turns out they waited to open their gifts for two weeks......this seems to be the trend now.

I like this idea but if this is a bed quilt, then I'd be tempted to make pillow shams to go with the quilt that is coming.

Handwrite a note and give it to them in a card. There are few things in this world nicer than a handwritten note. Explain that their gift will be waiting for them when they return from their honeymoon and in plenty of time to help keep them warm in the coming winter nights. Most people would understand. And, there is a chance they wouldn't be opening gifts until they returned from their honeymoon anyway!

I like NJ Quilters idea of including a few snippets of fabric in a card. I would just state the gift is not quite ready and they will have it as soon as possible. My aunt used to embroider a tablecloth for each bridal shower gift. The first couple were not done on time because she had heart issues, so she wrapped the unfinished cloth with a note saying she would take it back and finish it. After she was well, she did this anyway - it was her trademark. Shortly after she died, her first granddaughter was getting married and for her shower, I continued the tradition of giving an unfinished cloth. Brought tears to many eyes. Sorry, didn't mean to ramble on like this.

I know that this is not quilting , but I have given knitted afghans with the needles still attached. Told my niece that she would have to return it to me if she wanted it completed....UNLESS she wanted to finish it herself...
She knew that "the black garbage bag with the bow on it" meant that it was an afghan. She was the 4th in the family to get an afghan for Christmas... She waited until it was completed. I mailed it as soon as it was done!
I knitted all the way on the 2 1/2 hour car ride to the party...I just couldn't get it done..but then took my time in finishing it so that I did not drive myself crazy!
She was happy!