Most attractions in the higher elevations will be cooler, so to beat the heat, visit the stuff along Wildrose Road, which is around 4000 feet and higher. There are some good hiking opportunities at the attractions there, primarily at Aguerrebery Point and a 4-mile hike starting at the Charcoal Kilns up to Wildrose Peak.
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If you're a AAA member, go pick up a Death Valley map. It's one of the better Death Valley map guides.