If you're in the area, consider the drive up to Crystal Mill from Marble. The trail is narrow, so you'll want to go on a weekday when you're less likely to encounter traffic from jeep tours and such. Otherwise, I wouldn't say it's terribly challenging--my wife drove up and I drove down, and neither of us have any real off-road experience. The only tough part was passing a full size truck that impatiently came down the hill and tried to pass us through a narrow section. Inches to spare with an RB! Had he just waited 100ft up the trail, it was wide and flat...
Crystal Mill itself is beautiful even with the wires that now hold the disintegrating wood structure together, and I once read it's the second most sought after photo in CO (after the Maroon Belles). The road continues on through the town of Crystal and winds its way to Crested Butte via Schofield Pass, or you can do a loop including Lead King Basin, Devil’s Punchbowl, and Sheep Mountain, but I haven't done either of those and can't speak to whether they'd be good in a van. While you're in Marble, be sure to drive up to the Marble quarry as well.
Another beautiful spot to visit is Alta Lakes above Telluride. The road is non-challenging, wide enough for passing in most places, and passes through the ghost town of Alta.
The scariest part is the narrow section right off the highway--wide enough for only one vehicle with a dropoff on one side--but again, not challenging. Like Crystal, shoot for a weekday--it's quite popular, and you'll see why. Great place to camp if you can find a spot, and bring a SUP if you have one!
And if you're in Telluride, you should of course visit Bridal Veil Falls.