Hólmafossar is another cascading waterfall flowing on a tributary of Jökulsá á Fjöllum amongst foliage. There were lots of wildflowers when I visited this waterfall. Getting here requires a fairly lengthy walk from the Hólmatungur car park (about 12km north of the west bank car park for Dettifoss), but easily doable in around two hours return. I also managed to view the falls from the other side of the tributary on which it flows though that way is unofficial.