Here's a few things I've learned about everyday Icelandic, in my very basic introductions to speaking and listening to people, that perhaps you should know when visiting. (I could be wrong on some of them, of course, so don't quote me- I start my Icelandic Level 3 class on Monday so I'm eager to get speaking more again, but then again, there's so much they don't teach you in textbooks.)
Hvað Segir þú, also spelled 'Hvað segirðu'; usually sounds more like 'Kvsayatheuh' all blurred together. This can mean so many things but it seems to depend on context. It means literally, 'What say you', or 'How are You', but it can also mean 'What Did You Say/Say That Again', 'Wha?', and my favorite, 'You Don't Say!'.
Bara, the amazing word for 'only' or 'just', but also thrown around like candy, as easily as Americans use 'like' or 'um'. See also: hérna, which conveniently also means 'here', but now means 'like' or 'um'. I think also sko fits into this category.
Takk fyrir síðast. An amazing sentence that doesn't have any similar use in English, but I love using it when appropriate. Literally means, 'Thanks for the last time I saw you!' Great for maintaining friendships and reminding someone what fun it was when you last went bowling/sang songs/got drunk together and made out/bought hot dogs a month ago together.
Ha. Said only with downward inflection. A good tool when you didn't quite hear something and you want it repeated. Has nothing to do with laughter.
Near Inaudible Inward-breathing Já: Yes, but said breathing IN instead of spoken out loud! I didn't hear this for a good two or three weeks but now I hear it from people all the time, like a little too often. I think it's a 'yes' that's more like a 'uh-huh' or a 'yup', than a 'yes, of course/definitely'. But then again there is also jájájájájá (x infinity), which I also love.
Keyrum í gekk. Great phrase for going on a trip, when you're excited to leave. Colloquially translates to: 'let's drive this shit!'