• You could make a loop through Rocky Mountain NP in the summer when Trail Ridge Road is open.
• I'll second the suggestion of Boulder to Nederland and suggest continuing north to Estes Park then back to Boulder via US 36.
Much more obscure and less crowded, once away from the metro area:
• US 285 through Fairplay to Poncha Springs Supercharger Station, then to Westcliffe, through the Wet Valley to Walsenburg then home on I-25. Many other variations on that route can be tried. If you like camping, Lathrop State Park near Walsenburg has RV pedestals with 14-50 outlets for overnight charging (camping reservations required in all state parks).
^ Spanish Peaks from the Hogback Trail in Lathrop State Park, a short distance from the campground.
• Or US 285 through Fairplay to Poncha Springs Supercharger Station, then south to
Great Sand Dunes National Park, back to Poncha Springs, US 50 to Pueblo or Colorado Springs. Again, many variations on that route if you want to stick to two lane mountain roads rather than get home on I-25.
^ Part of Great Sand Dunes, with the Sangre de Cristo Mountains in the background; some of the dunes are more than 700 feet high.
^ A picture from about halfway up one of the dunes, to give a sense of scale.
If you are willing to get farther from home, my area in Ouray County is among the most scenic in the state.
Black Canyon of the Gunnison National Park, near Montrose, is well worth a visit. US 550 from Ridgway to Ouray, over Red Mountain Pass, to Silverton to Durango is very scenic. The part from Ouray to Red Mountain Pass can be a bit daunting for those uncomfortable with narrow roads, steep dropoffs and no guardrails (so that avalanches can flow over the road). Fun driving, however.
^ US 550 between Ouray and Red Mountain Pass (11,018 feet).