I never made it to Yellowstone.
Late September 2015 - second attempt. Success! I had three marvelous days in Yellowstone (not enough for this amazing place!). One of my must-see destinations was the Grand Prismatic Spring.
Most people see the spring from the boardwalk. It's interesting; it's nice.
But, if you want that classic view, you're going to have to climb for it.
Drive down the road a bit further to the Fairy Falls trailhead. Hit the trail. Go across the bridge and after a ways, you'll see a couple of rough trails up the hill to the left that overlooks the spring. It's rocky and a little steep but quite doable for the generally fit. And, it's SO worth it!
If you are up for some more walking after Grand Prismatic, complete the full Fairy Falls trail. It's about 5 miles RT and quite flat and easy with a lovely waterfall at the end.