K.P.
Her married monogram on a beach tote would be perfect! Along those same lines (depending on what you want to spend), you could purchase a piece of their luggage and put some honeymoon items in there- that way it is creative and off her registry!
When we got married, my husband's aunt knew that we were going to Rhode Island for our honeymoon, but that we were delaying the trip for a year (work schedules). So... she (and her daughters) bought us the carry-on for our luggage set and filled it with sunscreen, a visitors' guide for Newport, gift certficiate to a beautiful restaurant there, gift certficate to have a beach-side massage, gift certificate to CVS to get pictures developed and a photo album for the pictures. When we (finally) took our trip, there was a bottle of champagne waiting in our room with a note wishing us a Happy 1st Anniversary from his aunts and a bottle of wine from my parents. I love creative people!