When you refer to Aspen as your "first stop", I hope you mean first stop in Colorado, there is no way you could make it there from SD in a day.

Your right, Colorado is a beautiful State and there is much to see and do. The weather could play a role in where you go and how you get there, such as Independence pass between Aspen and Twin lakes might be iced up and/or snowed in, but no big deal just head out the way you got in via Carbondale. Rocky mountain NP is a favorite, even if Trail ridge Rd is closed you could still go to Bear lake etc near Estes park. Near to Pikes peak is Manitou Springs and the 'Garden of the Gods' and 'Cave of the Winds'. On the way to the Pikes peak toll kiosk is the North pole and Santa's Grotto that might interest your young one. From Canon City you could continue West on US 50 through Currecanti Rec. area and Black Canyon. From Montrose head South on the 'Million dollar highway' through the mountain towns of Ouray and Silverton to Durango, maybe take a ride on the mountain train.

Once you have nailed it down some more, I am sure we can help fill in the gaps.