We also stopped off in Island Park on our way to Yellowstone. We got there as the sun was going down and it was getting chilly so we didn't hang out for very long. The girls loved feeding the fish and nibbling on some bread of their own.
It was absolutely beautiful there. I would love to go and spend some time in a cabin up there. Escape the business of the real world and enjoy the natural spring river, go check out the lake and just hike around.
I think that will be something we add to our bucket list. It is also really close to Yellowstone so you could explore both the National Park and the Island Park area.