If I were staying in Vegas, mid-May, I'd research several surrounding options and then keep an eye on weather forecasts and snowpack conditions once May rolls around. Telescope isn't my reason for that; it will very likely be great at that time and is worthwhile. However the rest of Death Valley might be blazing hot and there are other great hikes and mountains that are of similar or lesser drive time from Vegas than Mahogany Flat.
I'm not sure if you've been before, but Zion National Park would be closer than Mahogany Flat, and not as hot as Death Valley. Nearby, this would provide a spectacular adventure off the beaten path: http://www.summitpost.org/canaan-mountain/363242
Charleston and Mummy, very near Vegas were mentioned. Also keep in mind that Lone Pine, CA and the Eastern Sierra are just beyond DVNP to the west. In a low snow year, hikes from Horseshoe Meadow Campground to the gorgeous Cottonwood Lakes or even peaks like Cirque or Langley might be possible (altitude and equipment needs may add complications, especially if summitting a peak is important).
If you luck out and hit a cool spell, Death Valley has great canyons and fun dunes. Many sites or hikes are short enough that you can see them without leaving air conditioning for long. Longer hikes might be doable early in the mornings, and at least it is a dry heat. Mosaic Canyon, Golden Canyon, Zabriskie Point, Artists Drive, Badwater, Dantes View, and the Stovepipe Wells Dunes are all quick and scenic. Marble Canyon, Fall Canyon, and Palmer Canyon are great mid range hikes away from crowds if it is cool enough. Pyramid Peak is relatively near some of the tourist sites mentioned, but be careful not to be there afternoon in a heat wave. Dry and Tin are a bit far out northwest and long hikes, but scenic, and you could stop by Ubehebe Crater, or even the Racetrack while out that way. Also out to the north, Eureka Dunes and Last Chance Mountain are interesting backcountry spots and you could even loop back along the Sierra via Big Pine/Big Pine Road if time permits. Besides the scenic driving along the Sierra, there is great hiking from Big Pine Creek, Onion Valley, and (again) Horseshoe Meadow.
Telescope Peak is likely to work out well for you that time of year, regardless of what else you are able to fit in. All of the lower peaks and hikes have a chance of being toasty, so be cautious and hydrated. I think 100 is average that time of year down low and the sun will likely be intense.