Depends on what and how much you want to do or dawdle between the two.

On that leg of the trip, I'm mostly wanting scenery rather than hiking et al., but if folks think there are "must stops" (towns or otherwise), it would be great to know.

Glenwood Springs is only 41 miles from Aspen. Right next to the Colorado River and I-70 is the lodge with the world's largest hot spring fed pool. http://www.hotspringspool.com/x.php?p=theLodge
You could also ride a gondola car up to the Glenwood Cavern which is also within walking distance. There is also a bus service that connects Glenwood with Aspen.The Glenwood Canyon east of town is very scenic.

First off, in the summer, it is shorter to Aspen via Leadville and Independence Pass and the scenery is terrific especially over the pass. However, return via Glenwood Springs and I-70 so that you go through Glenwood Canyon. Aspen is not that far so you would really have to dawdle to stretch it into two days. Aspen is expensive even in the summer. It is about a 45 min drive between Aspen and Glenwood so I don't know if that is exactly the best option. Leadville has some interesting things including the Mineral Belt trail. It is a hard surface 12 mile trail that takes you into the old mining territory. Reasonably level for the mountains. In between you have Georgetown and the Georgtown narrow gauge loop railroad. There are lots of things to see and do.

The loop sounds perfect -- thanks. As does the gondola ride. I'll copy down the info on both!

But I believe you will want to drive through RMNP on Trail Ridge Road and then drop back down to I70.
Leadville is a terrific little town.

Trail Ridge Road is probably the best, but if you can drive the Old Fall River Road too. Both are very good.

Thanks for the tips. We'll be staying in Estes Park and leaving from there, if that makes any difference. We'll be plotting this out on a map very soon.