Took my kids and on a 3 week trip a few years back to Yosemite- absolutely gorgeous! have been thinking about Yellowstone, been looking at taking the "Empire Builder" (northern route) on Amtrak out of Chicago, getting off at Glacier Park, MT, then driving south to Yellowstone, and continuing to Denver to fly out. Empire route is supposed to be very pretty (although not sure of the car rental rates one-way from Glacier...).