What do your kids want to do. Maybe they would really like to ski, even for the rest of the time somewhere else.

Las Vegas isn't close to Park city, anyway, it's over 400 miles away, so I'm not clear on the choices. That would take up almost an entire day to drive that distance. I don't find Utah that fascinating myself except for the skiing and national parks, so wouldn't personally want to spend that much time in Utah cities (like SLC, been there a couple times, it's good for a day or so).

The problem is Park City isn't really super close to anything, I suppose I'd go to Colorado. Durango is about 500 miles away, but that's not too much farther than Las Vegas was, if you are contemplating driving that far. So is Denver, of course, but you could have more weather problems that way, not sure. All the neat places I can think of in CO would be for skiing (like Telluride). But there are some things to see around Durango, not sure how it would be in March. new Mexico would be nice but that's even farther, like a 10 hr drive (say to Santa Fe).

I suppose the closest place would be Idaho, like Pocatello or Twin Falls or IDaho Falls. Ive never been there and March probably isn't the best month, but maybe look into what you could do there. https://vacationidea.com/destination...-in-idaho.html

Head to Escalante and hike in Grand Staircase National Monument. Do NOT head to Idaho (I am from Pocatello-nothing to do there in March). The desert will be your best bet if you don't want to do snow activities. You could also go to Bluff and do some amazing hikes with Far Out Expeditions--great guide for day hikes. You will have to drive 4-6 hours to get anywhere. That's the norm. You could check to see if there are still snow cat trips to Yellowstone---that is an amazing experience!!! You could hop a short flight to Phoenix or San Fran or Denver if you wanted to do that.

Take the Amtrak California Zephyr east from SLC to Glenwood Springs CO. The train leaves SLC about 3:30AM but by sunrise is through Green River and heading to the Ruby Canyon. Ruby Canyon is one of the most beautiful sections of the CZ route with high cliffs on the north side and the Colorado River on the other. There is a 10-15 minute stop in Grand Junction CO where you can pick up some things you can't find on the train (ice cream sandwiches, peanuts in shell, etc). The train arrives in Glenwood Springs about noon (8.5 hours).
There is a cavern to explore and you can hike up to the cemetery to see the memorial to Doc Holliday. There is a good bus service from Glenwood Springs up to Aspen if you want to visit another ski resort.
You can take the CZ out of Glenwood Springs a little after noon each day. The 20 miles or so east of Glenwood Springs are another beautiful view of the Glenwood Canyon with out having to concentrating on driving on I-70. There is a good new light rail service from the newly remodeled Denver train station out to the Denver airport.

You are not going to find much skiing Glenwood Springs and even less at Ft. Collins. Forty miles south of Glenwood is Aspen maybe a little cross county near Estes Park that is somewhat close to Ft. Collins. If skiing in CO you need to stick to Summit Cty/Vail or even Winter Park. But it is almost an all day trip from Park City to Colorado skiing.

Yellowstone road close to oversnow travel between early to mid-March https://www.nps.gov/yell/planyourvisit/parkroads.htm

There's not much in Pocatello, Twin Falls or Idaho Falls. Most of the scenic areas of Idaho are farther north though we did enjoy City of Rocks in southeastern Idaho but I'm not sure what it's like in winter.

You have been just once to the national parks and have seen enough? Wow.