The area around Varmahlíð is home to northern Iceland's best white-water rafting. Trips run from about May to September.
Both Bakkaflöt and Viking Rafting raft on the high-octane Austari-Jökulsá (East Glacial River; Class 4+ rapids) and the more placid, family friendly Vestari-Jökulsá (West Glacial River; Class 2+ rapids).
Drysuits are provided. Check websites for up-to-date schedules and prices.
There are half a dozen companies in and around Varmahlíð, including Hestasport and Lýtingsstaðir offering shorter rides (one to two hours) for beginners, plus longer day outings. A few plan week-long expeditions into the highlands, or offer the chance to participate in the annual sheep roundup (réttir) in September. See websites for programs and prices.