Hiking -- Ferðafélag Íslands (tel. 568-2533; www.fi.is), Iceland's premier hiking organization, leads a couple of multi-day trips around Kjölur each summer.
Horseback Riding -- Two of Iceland's most reputable stables run Kjölur tours on horseback. The Íshestar (tel. 555-7000; www.ishestar.is; 166,250kr/$2,660/£1,330 per person) tour is 9 days, while Eldhestar (tel. 480-4800; www.eldhestar.is; 148,750kr/$2,380/£1,190 per person) tours are 8; both give you 6 days in the saddle. Food, accommodation, and transportation to and from Keflavík airport are included.
The Kjölurvegur Trek -- The Kjölurvegur (aka Kjalvegur) is a rewarding and very manageable 3-day hike along an old horse trail from Hvítárnes to Hveravellir, with mountain huts spaced at 4- to 6-hour (12-14km/7 1/2-9-mile) intervals. The route skirts Langjökull, with interesting but optional detours from Þjófadalir hut east through Kjalhraun lava field or west toward Langjökull. Buses stop at both ends of the Kjölurvegur, though you'll have to walk the 6km (4 miles) from Route F35 to the Hvítárnes hut. (Reserving bus seats in advance is a smart precaution.) Online, www.fi.is has a day-by-day breakdown of the route -- under the heading "Hveravellir-Hvítárnes (the Old Kjalvegur Hiking Trail)" -- but it takes you from north to south. Hiking south to north lets you end the trek with a valedictory dunk in Hveravellir's geothermal hot tub.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.