The 250 miles between these two criminally overlooked parks comprise one of the best road trips in the country, offering Mars-like landscapes, eclectic pit stops, and almost complete isolation. Take a week to cover it. Monday: Slip the masses in Carlsbad Caverns’ Big Room by signing up for a three-hour guided tour of Lower Cave, where you’ll climb slick flowstones and explore eerie formations like the Texas Toothpick ($20). Spend the night at the Trinity Hotel, a refurbished 1892 bank (from $149). Tuesday: Drive 175 miles south to the McDonald Observatory, a cluster of mammoth telescopes perched on twin 6,000-foot peaks with outstanding views of Saturn this month ($5). Bunk at the 39-room hilltop Indian Lodge at Davis Mountain State Park ($95).
Wednesday: Hike seven miles of rugged trail in Limpia Canyon Primitive Area to views of the Chinati Mountains. Then get back on Highway 118 until you hit Terlingua, just outside Big Bend National Park. Check in at La Posada Milagro, built from the stones of a ruined mining town ($195), and grab pork spare ribs at La Kiva (lakiva.net), a bar carved into a cavelike space on a dry creekbed. Thursday: Explore Big Bend’s 150 miles of hiking and biking trails, ranging from 7,000-foot peaks to desert slot canyons. Friday: Float the Class II–III Rio Grande through the 1,500-foot-deep Santa Elena Canyon, a sheer limestone gorge patrolled by peregrine falcons and wild burros (raft and canoe rentals from $35; guided trips from $70).