D.B.
You could do a time capsule, but you'd have to figure out where to bury it - if it's on the property of the old house, you'll have to be sure that the new people will let you come back in a year or 5 to dig it up.
You could have each person write up a favorite memory, complete with sketches or photos, and you could create a "house scrapbook" with those things.
You could also have a farewell dinner - maybe a picnic on the family room floor or someplace else where you all spent a lot of time (deck, basement play area, whatever). Have everyone share the memories they've written down. If anyone's really creative, you could create a poem or a song parody (re-write a famous song - "My Favorite Things" is a tune everyone knows and it lends itself to a list). Take photos of this picnic and make it the last page of the house scrapbook. Toast the old house and the memories with champagne (sparkling cider for the kids) - just as you say goodbye to the "old year" you can say goodbye to the old house, combining fond memories and a few tears with the excitement of the new space.
Since you are staying nearby, your current neighbors may not throw you a farewell party but you can certainly invite them to this picnic or pot luck.
You could leave a little note for the new buyers about how great this house and neighborhood have been for you - either give it to their realtor, or invite them to this get-together, or even tape it to the mirror in the main bathroom or to the front of the stove - some place that won't be in their way when they move in, but where they will see it.