Diablo Lake and it's brilliant turquoise color is breathtakingly beautiful. If you are in the North Cascades it is not to be missed. If you are not up to hiking there is a fantastic scenic viewpoint just off highway 20, east of the lake.
This place is beautiful! A North Cascades ranger recommended a short trail that contained the oldest and largest trees of the park. It was beautiful and it was great to walk in such an ancient forest, amongst the majestic old growth. I would highly recommend. The park also has ADA-accessible hiking trails.