If you can, get out to Skogarfoss, one of the most iconic waterfalls in the world. You’ll know it when you get there- remember the Scottish Widows advert? On the way to Skogarfoss you’ll pass Seljalandesfoss, what a place.
The Golden Circle Tour is really interesting but obviously touristy- if you can find a way to do it independently, either via public transport or via a hire car (GO ON, it’s your birthday!)you will get to see Geysir, Gullfoss and all the other amazing places in peace and quiet and at your own pace. And get to miss out the tedium that is Hveragerdi and replace it instead with an hours walk up to bathe in the hot river pools.
The sea food is sublime, the beer expensive. The locals are affable, the scenery is stunning. It’s great to stay in town but staying slightly out gives a better experience. We’ve stayed in a few places but the best has always been Hotel Floki on Snorabroaut/ Flokagata. Sitting in the garden, drinking a beer with some Greenlanders jammin’ on an accordion and some drums takes a bit of beating.