ideas:
Decorative clear-glass vase table centerpiece
Donate to a middle school for games / learning how to play marbles
Put them in a fish bowl. Add water and goldfish. Beautiful!
Add them to a water feature / fountain / pool as a fun & unexpected prize for kids to find.
Crush them and add as a layer to the top of your new sidewalk for an arty touch.
Actual Marble marbles? Epoxy them together into a scaled-down statue of David
Glass marbles? Cement together for quasi-stain glass windows and use as garden art (or actual windows)
Drop them all in the gas tank of your enemy.
Leave them in random places, slowly, all over the city (grocery store shelf, walk signal button, tip jar, at the gas pump...) and, day by day, make someone's day.
or
Let them be free: roll them down Denny. The bad guys from Home Alone will likely appear, mid-escape from some heist, and you'll have saved the day ;)