I'm pretty sure you'd know if you were in this area, but anyway: be sure to take a walk through
Hallormsstaðarskogur, Iceland's largest forest. If you get lost, just stand up. :) But seriously, it's a very pleasant stroll if the weather is fine.
Just across Lagarfljót/Lögurinn is Hengifoss, which, at over 100m, is one of the highest waterfalls in Europe, I guess. The hike there takes about an hour (possibly more if you're not used to climbing steep hills) from a nearby parking lot; be sure to wear good shoes as you have to climb over rocks and jump streamlets as you get higher up (it looks like a deceptively smooth walk in the beginning). Also prepare for strong, wet, biting, cold wind, even in high summer.
The links link to pictures we took at the linked locations.
