Collection of Stones
This stone collection in a jar was used as a unique wedding guest book. Guests would write their well-wishes for the wedded couple on a smooth stone, then place them in the glass vase.
But this idea could easily be used to memorialize your trips as well. Take a stone from every city you visit and write your favorite memory or quote from the trip as well as the dates. Plunk them in a jar and watch your collection grow.