My response is a little late but we've been out enjoying Colorado!
First, if your trip is scheduled for mid-July, be warned that many of the NP's in UT are quite warm that time of year. Since you say this is the first trip of your retirement, I'd breeze through Utah and plan a return trip just to the UT parks in either the spring or fall when the temps are much more forgiving. One can easily spend weeks/months just exploring southern UT and doing so when the temps aren't so oppressive makes for a much nicer experience.
Mid-July in CO however is another matter. The snow has melted, the flowers are out, and the trails are dry. I'd spend quite a bit of time exploring the mountains at that time of year. Others have mentioned US 50 as an option across the state and indeed that is a nice drive. CO NM and Black Canyon of the Gunnison both are worth some time to explore. From Gunnison, do take a day trip north though Crested Butte to the little "town" of Gothic. The wildflowers around Gothic are just amazing in July. Further along US 50, the town of Salida is worth spending at least a few days as a base camp to explore the upper Arkansas River Valley up to Leadville. Hiking, rafting, biking, 4x4 trails, fishing, rafting ghost towns -- all of these things are found in that area. You can take US 50 all the way to I-25 in Pueblo and head north from there. Or cut off on CO 115 at Penrose and head to CO Springs that way. Or head north from Salida on US 285 and catch US 24 heading out of Buena Vista and take US 24 into CO Spgs.
But there are other options worth mentioning. Even if you take I-70 through UT, you can continued on I-70 in CO and turn off on US 24 at Minturn. This will take you through Glenwood Canyon on I-70, arguably one of the most scenic stretches of interstate in the country. There are several rest areas in the canyon where you can get off, park, and walk down to the river or take a short hike. Once on US 24, just head south through Leadville and on to Buena Vista or Salida (as discussed above). Along the way, you'll pass Camp Hale, home to the 10th Mountain Division during WW II. Worth spending an hour or a day exploring if that sort of thing interests you. Leadville is an old mining town nearly two miles high. Lots of historic buildings and full of characters.
Yet another option is to take US 89 to AZ 98 to US 160 and take that across the southern part of CO. Plan on several days in the Durango area. Mesa Verde NP is worth a couple of days. Don't miss the Durango-Silverton Narrow Gauge RR. Not your typical tourist railroad. Unbelievably scenic (and this is coming from a Colorado native). From Durango, take a day trip and loop up CO 145 to Ridgway and back down US 550 to Durango. Even if you take the train, a separate day trip to Silverton will allow you to explore the town and the area with more leisure. We try to spend 4-6 days a year just in the Silverton area. Don't know what your toad is, but if not 4x4, rent a jeep and explore some of the easier trails around. American Basin over Cinnamon Pass has amazing wildflowers by late July. After leaving the Durango area (if you can pull yourself away), you'll head over Wolf Creek Pass. You'll eventually pass Great Sand Dunes if that interests you. You can either continue all the way to I-70 and head north, or turn north on US 285 and go over the very easy Poncha Pass which will also take you into Salida (catching a theme here?) and on to Colorado Springs from there.
Anyway, there are several options for you. Partly depends on how much time you have and what you like to do. I would look at reservations for some of the more popular destinations, as mid-summer is very popular in the cooler elevations of CO (everyone is trying to escape the heat).