Definitely Badwater - just to feel the burn! Maybe they'll luck up and hit a 125-degree day - braggin' rights back home. As they make their way down Badwater Road, they should certainly drive the Artist's Loop as well. The Devil's Golf Course is along here too. Other summer sightseeing would be driving up to Dante's View - best view of the valley in the park by road access, but it's a bit out of the way. Otherwise they could just hit Zabriskie Point on their way in on 190.
As they're driving 190 they'll also pass easily-accessible historical sites like the Twenty Mule Team Canyon and the Harmony Borax Works. And certainly the sand dunes at Stovepipe Wells.
Detouring up Wildrose Road to the charcoal kilns not only gets you out of the extreme heat for a bit, but offers a nice view of the Sierra. The only two maintained trails in the park are in this area (Wildrose and Telescope Peaks). Takes at least 30 min each way from 190, though. They could also hit Skidoo if they took Wildrose Road.
If they really wanted to detour north they could visit Scotty's Castle and Ubehebe Crater- but that's an extra 45-50 miles each way. Plus, Scotty's Castle has a seperate fee to tour the home.
By far, the best souvenirs, etc. are at the Furnace Creek General Store. Stovepipe Wells is the little brother in this category. The Panamint Springs Cafe has an incredible beer selection - considering. Most people will never have heard of 70% of the beer inventory they stock. I think that's the name of the place - either way, it's the only watering hole in Panamint Springs.
Unfortunately, they'll probably come away with a July impression of this place being hell on earth, and to be avoided. It's a completely different world in late fall or early spring.
