My wife and I started taking our grandson (now 12) on educational trips when he was only 4 and have continued every year. Since your grandkids are school-age here's a list of places we've taken our grandson during his elementary years:
Disney World, The Smithsonian, Boston, Arlington, Colonial Williamsburg, Philadelphia, Jamestown Va, Pike's Peak, Yellowstone four times because he loves it so, Mammoth Cave Ky, Grand Canyon, Niagra Falls, Strasburg Pa. train museums, zoos in Cincy, Louisville, Washington DC and others, and we've taken him with us on our last two cruises to the Bahamas and Mexico. He's been many other places but I don't want to hog the thread. Hope you get great ideas and have as much fun as we're STILL having with our boy.