Lot's of really nice places, how about...
From Yellowstone, drive through Quake Lake and the Earthquake Interpretive Center
https://www.fs.usda.gov/detail/custergallatin/specialplaces/?cid=stelprdb5127785, through Virginia City, and camp at Bannack State Park
https://fwp.mt.gov/stateparks/bannack-state-park. Spend the day and tour the ghost town, do some hiking and bird watching and rest up from Yellowstone.
From Bannack State Park, cut up to Wise River along the Pioneer Mountains Scenic Byway
https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5052331.pdf and camp along the Big Hole River. Along the way, stop at Crystal Park National Recreation Area
https://www.silverbowclub.com/dig-for-crystals-in-crystal-park/ and dig for crystals.
From the Big Hole, I would head west, visit the Big Hole National Battlefield
https://www.nps.gov/biho/index.htm, and from there...head over to Lost Trail Pass and camp on the Bitterroot River between Sula and Darby.
Leaving North up the Bitterroot will take you up through Missoula...From there I would head up US 200 along the Clark Fork River towards Noxon and camp at one of the place near there. The goal would be to set your self up to visit the Ross Creek Scenic Grove Giant Cedars
http://www.libbymt.com/areaattractions/rosscreekcedars.htm, walk the Kootenai Falls Suspension Bridge
https://www.themandagies.com/kootenai-falls-swinging-bridge/by Troy Mt. and head for Glacier National Park.
Lot's of things to do in Glacier....take your pick but be aware that Many Glacier Road may be under construction and you might consider a jaunt into Canada to hit Waterton Lakes National Park
https://www.pc.gc.ca/en/pn-np/ab/waterton/.
Finally, after visiting Glacier, I would head down the Sealy Swan Hwy
http://enjoyyourparks.com/Seeley-Swan-Valley-Montana.html and camp at Clearwater Junction. From there, I would spend the next day going up the Garnet Range Backcountry Byway and visit the ghost town of Garnet
https://crownofthecontinent.net/entries/garnet-range-back-country-byway-montana/6a76d698-5f71-4405-8066-f242f4870bcf.