You’d be hard pressed to find any place in Washington state more rugged than North Cascades National Park and its surrounding area. The mountains jut straight up out of the earth, no foothills or gradual incline to speak of. The dramatic landscape makes many of the area’s trails among the most challenging you’ll find. Luckily for those not looking to set their thighs on fire or to take a mallet to their feet, the 7.5-mile-long Diablo Lake Trail is an easier option that nevertheless leads to some beautiful views.
Note that I said “easier” here, not “easy,” which is what some other sites designate the Diablo Lake Trail as. I disagree with that designation, and I base that disagreement on the fact that most of the people I came across on the trail were sweating heavily and asking how much farther they had to go.

I’m just getting back into hiking shape myself, but I’m not THAT far out of shape, and the trail certainly didn’t feel “easy” for me, just “easier” than the other trails I’ve done in the North Cascades. It’s no Storm King, but it’s no Box Canyon Loop, either.
I’m more apt to lean towards a classification of “moderately difficult” in terms of strenuousness. The Diablo Lake Trail is straightforward, though, and doesn’t have any river crossings or climbs or anything that will test one’s outdoors skills in any serious way (assuming that you don’t sustain some kind of injury, of course).
The trail takes you up the mountain and then down the other side to the shore of Diablo Lake. The lake is artificial, created by Ross Dam, which you can also see while being down by the lake. So, if you’re looking for a feeling of escaping civilization, this trail might not be for you.

If you’re just interested in a nice, easier hike with some pretty views, the Diablo Lake Trail is a good choice. It’s not as as easy (and not nearly as short) as something like the Skagit River Trail, however, so there may be better options if you’re looking for something extremely quick and easy.
The trail leads through the base of a boulder field that offers some interesting views. Another section of the trail leads over an extremely steep slope that drops off and leads a long, long ways down to the water at the bottom of the mountain. The trail is plenty wide at this spot, but people who don’t like heights might find it frightening.
North Cascades National Park is one of the lesser-visited parks in the Pacific Northwest, but its spectacular, rugged scenery shouldn’t be missed. The Diablo Lake Trail is a good starter hike to get to know the area and to get a feel for what it has to offer.