This may be too late, but if they're driving thru Green River, Utah on I-70 the Powell Musuem might be a place they would enjoy since he opened the Grand Canyon to geological exploration. There's a nice state park right on the river in Green River which is a good place to camp, although sometimes the mosquitos can be bad there.
If they want a quick, close look at the Colorado River, exit at Loma, south over the bridge and left on the dirt road half mile to the river. Lots of boaters put in there. It's really cooking with the snowmelt right now.

Cheapest fuel in the Vail Valley is the Eagle Vail Cafe, exit just past Vail for Minturn and continue west on the frontage road for about a mile. It's on the left down a steep driveway, a little hard to see. Don't know if they have diesel but it is the best bakery within a couple of hundred miles.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13