Travelling east from Reykjahlíð towards Egilsstaðir, the Ring Road crests Námafjall ridge, passes Hverir (mind the stench!) and offers a turn-off to Krafla. After another 20km you'll encounter the turn-off north to mighty Dettifoss via Rte 862 (sealed), and then the 4WD-only Rte F88 turn-off south to Askja in the highlands.
Soon you'll cross the bridge over the glacial river Jökulsá á Fjöllum; Rte 864 is signposted north from just east of the bridge; this rough gravel road is generally open June to October, and arrives at Dettifoss (eastern vantage point) after 31km, and Ásbyrgi after 60km.
From the bridge, the Ring Road takes a short cut inland across the stark highlands of the northeast interior. If you won’t be travelling into the highlands proper, you’ll glimpse their start here. The ostensibly barren, grey-toned landscape is dotted with low hills, small lakes caused by melting snowfields, and wandering streams and rivers.