and all by car yes? The "devils tower" is fantastic, easily the most peacefull sereane place ever visited.. AND on the way to Yellowstone, just to the East...
In Utah I love the desert.. moab for arches would be a WONDER, equally as unique as GC and YS, not as big, BUT the larges collection of arches etc etc.. the drive down 191 is ok... IF you go take the westward little dirt forest access road to some point??? about twenty miles south of the entrance for canyonlands.. more or less.. its some point.. but it has this unbelievable view, and few folks are ever there!
also south along 191 is the four corners area.... three weeks is doable... and I would see brice and zion coming or going ... if you do not want to spend time on water skip lake powel and flaming gorge till you are here in the west again. ruins? top picks again in order of preference, Chaco canyon, Canyonlands, Mesa Verde,
cliffs? Grand canyon X10, Bryce, Zion, Yellowstone.. but YS is a trip all on its own... for sure you can camp in very very nice areas.. I liked every time there at different sites... but once there see mamouth, mud pots, and old faithfull.. LOTS of wildlife, Elk, Buffalo, Bears including Grizzlies, ... no other thoughts off the top of my head.. Independance pass, very cool drive... but most of CO in the mountainous area is beatifull. After Mid may the road to the top of Mt Evans, paved to top at over 14k is special.. and free, ignore the us dudes askingfor money, they only can charge for the services, and not the road use! one win for the state of CO against Bigbro! lol very nice to see the setting sun and lots and lots of starts.. at 14k and away from the city makes a great stop.... again co has lots of great mountains to see, BUT you are limited so given you spend time where you find the best oddities.. also the badlands are cool... R u flyining to Denver and getting a rental car? That does affect some of the recommendations...
HAVE FUN WITH THE KIDS! sounds awesome!!
fyi, my wife and I moved here, CO, from the Boston area in 1993 and she had never seen the west, so we did several two and three week trips in the first three months after unpacking.. all the way to WA, lots to see... out of my many many miles in the west... there may be no better stops that Yellowstone/ Tetons and Grand Canyon... many different.. but no better and they are popular for a reason....