Just a suggestion, but while you are having so much fun with them, be take some pictures! Then, if you pull out the arts & crafts things, help them each decorate a picture frame. It can be as simple as a cardboard frame that will fit around a 4x6 snapshot hung on the refrigerator or as elaborate as a cheap frame from a craft store so the kids can glue things on them & stand them up on their dresser at home. Once your pictures are in, you could mail them their completed frames as a momento of your special weekend. Woudn't that be special? Come to think of it, have them help you make a third one so you get to keep one too!
Just be sure to put down a plastic tablecloth or something underneath, because a an Aunt to 9, I can tell you how messy even the simplist project can become! But those are the most fun, of course!!!!
No matter what, enjoy their visit,
Nina