Long challenging day hikes

Got a Vacation coming up and I'm still collecting all my lightweight gear. So instead of backpacking, I am looking to still get out on the trail and hike.

I'm looking for ideas for long day hikes. Id prefer loops but anything could work. Something like the Grand Canyon Rim to Rim or Mt. Whitney. I from Michigan so the closer to the Midwest the better. I'm kinda thinking the Colorado area is where I will be heading. Any good websites? Id like to find a place where there are a couple of trails with in a decent driving time.

Any input or ideas is appreciated.

Thanks
Andy