This was a smoke day classified as ‘moderate’ by EPA’s smoke website to give you an idea of conditions here when you see that rating.  I've added a short video of the hike below..

Smoke:  from the viewpoints we could only see the outline of Pyramid and Colonial peaks.  Same with Sourdough scross the lake.  No breathing trouble though and I have asthma.

bugs: yellow jackets at the viewpoints were a pest

wildflowers: only some pearly everlasting, hardhack and fireweed.  

Wildlife: saw three teenage spruce grouse chicks and thier mom!

Decided to tack on this hike after a morning of paddling the lake, to make the long drive from Seattle a bit more worthwhile. This was my second time at Diablo, parking at the Colonial Creek Campground to use the boat launch there. Was not nearly as busy as the previous time (late June), there was parking (not ample, but available) the whole time I was there from 8:30-2:30. I will say traffic was heavy leaving the park in the afternoon, and the drive back took a full 45 minutes longer than the drive in, so keep that in mind if you're trying to make it back to the city by a certain time. 

Started up at 12:30, reached the top about 1:15, rested and snacked for about 20 mins, and was back to my car a little after 2:00. I went at a moderately fast pace, stopping to take pics. The start of the trail is rocky (large gravel) and hard to find at first, but once you cross the bridge leaving the campsites the trail is easy to follow from there, and mostly dirt with few rocks/obstacles. Plenty of places to stop and rest on the way, up as well. Trail is mostly OPEN and not shaded so it was a hot one, definitely don't forget sunscreen. It's not too steep or long, though, so that was a benefit given the temps. Not very busy either, which was nice. I passed other hiking groups once every 10 minutes, max. Some flies and other bugs, but not as many as expected. Bug spray still recommended if bugs tend to like you.

Certainly recommend if you want a short, not too strenuous hike in the area with GREAT views. The view of the lake isn't as good as the main Diablo Lake trail but they are still worthwhile given the trail is half as long, and the views of all the surrounding peaks are excellent, at multiple points both up and down.  

Final random note--I stopped to splash my face and head at the end, to rinse off sweat/sunscreen/bug spray and to cool down in the stream right as you re-enter the campground. Once my hair dried in the car, it was so unbelievably soft. I don't know what's in that water, but ladies (and guys) I recommend sticking your head in it ;) 

After packing up our campsite at Colonial Creek, we arrived at the trailhead just after 9am. As described, the signed trail starts in a dry riverbed in the campgrounds, though the day use parking lot is right off Hwy 20. Once you reach the forest, the trail is wide and very well maintained with a fairly gentle grade. There are a few benches along the way, and several at the top. The viewpoints for Colonial Peak and Diablo Lake are just stunning. We had the hike up and the top of Thunder Knob all to ourselves, started running into ascending hikers once we were about a third of the way back down the trail. 

Well traveled and somewhat crowded in spots.  Beautiful views, especially overlooking the lake.  Don't expect solitude- since it is one of the easiest and shortest hikes in the N. Cascades, it is well traveled by adults and children.  

A steady, gradual climb with lots of switchbacks and plenty of benches for resting.  Dog friendly!  Many other (leashed) dogs on the trails.

There were a few flies at the top of the trail but nothing horrible.  Parking is a bit of a problem on a busy day- sometimes you have to wait it out for somebody to move, but people were leaving and arriving in pretty regular intervals and it only took about ten minutes to get a spot.