The Comet Falls Trail is a very popular hike in Mount Rainier National Park, and for good reason. Not only are the falls some of the tallest you'll find in the park,* the trail leading to the falls is a beautiful hike that is perfect for a half-day itinerary. In terms of the return for your modest effort on this hike, the Coment Falls Trail is hard to beat. You'll have intimate views of Van Trump Creek and its abundant cascades that are carved into the granite stream bed, and the sound of the creek is never out of earshot as the trail climbs through the protected forest of noble firs and yellow cedars. Keep a watch for Bloucher Falls at the confluence between the East Fork and the main stem of Van Trump Creek, and don't forget to look up occasionally for views of Mount Rainier's summit as the mountain briefly emerges from over the canyon.
With so much going on in such a modest distance, and given that access to the falls is seasonally limited by snow (unless you have snowshoes), you might expect some crowds for the Comet Falls Trail. You'd be right. The parking area at the main trailhead is often full, and you shouldn't expect to have the trail to yourself in high season. That said, the falls are a beautiful destination, and it is well worth planning a visit early in the day or mid-week when crowds may not be so intense. An alternate route to Comet Falls can be strung together via the Wonderland Trail that leaves from the Longmire area, as well. Note that dogs are not allowed on trails in the national park.
* With a total drop of 462 feet and its tallest drop of 392 feet, Comet Falls is Mount Rainier National Park's tallest waterfall accessible by trail. At 354 feet, Spray Falls is the second tallest accessible waterfall. Margaret Falls, dropping an impressive 1,350 feet from Cowlitz Park is the park's tallest waterfall, but it is all but inaccessible to the overwhelming majority of visitors to the park. Fairy Falls, which drops 680 feet from Williwakas Glacier (just below Paradise Glacier), is the second tallest, but it is also inaccessible via established trails.