(Original post by CHY872)
Ok. I've had two holidays in Iceland, staying for four weeks in total. My itinerary was very different to yours (I travelled around a lot), but I reckon I've still got some useful tips for you.
Firstly, the Blue Lagoon is a little disappointing (at least for me). If you read the publicity you are told it's some kind of natural spa with magical healing properties -- whereas really it's the outflow of a nearby geothermal power plant, which is so mineral rich it goes milky blue. So really, it's a €20 dip in a swimming pool with too many bath salts added. In any case, the Icelandic swimming pools are really nice anyway (really warm).
The Golden Circle is nice enough -- a good day trip, and do-able from Reykjavik. Fun fact: The prime minister of Iceland lives in Thingvellir. It turned out the nicely kept garden we were wandering through was actually the Prime Minister's garden. Oh dear.
Whale watching is amazing (though I did it at Husavik). Had a whale dive right under the boat. If you want to try eating Whale, that's pretty good as well (but don't have whale mincemeat, cos it tastes crap (to anyone who's opposed to the killing of whales (which ironically includes me) I just did it to try it. I will not be eating whale again.
Glacier walking and climbing -- go for at least like moderately challenging routes. The easy rated one is designed so that (literally) everyone and their mums can do it, which is kinda boring.
I'm not sure how much I'd recommend ATV riding (though I haven't done it). I just don't see how unique the experience would be, since you can do it fairly easily in the UK. I'd definitely recommend snowmobiling though, which is run by all the major tour companies (there's a shop that organises all the tours near the big cathedral in the centre of Reykjavik (when you get there, you'll know the cathedral I mean).
I'll give a massive recommendation for Icelandic Fish and Chips, a shop in the harbour: http://fishandchips.is/
It's not *quite* the best fish and chips I've ever had -- but it's the best I've had in iceland, and is just really good.
Also a must try is the hot dog stand in the centre of Reykjavik (google it). The food's not amazing, but it's definitely better than cheap McDonalds in the UK and costs like £1, which means you can buy 10-20 hot dogs for the price of a standard meal at most non-Reykjavik hotels. Bargain.
And you're paying less than £10 for a meal there. Expect to get fleeced if you eat out at most places. Don't buy alcohol if you can avoid it.
Try Egils Maltextrakt (comes in a beige 500ml can). It tasts like a slightly sweeter kind of beer, with no alcohol. It's the first type of non-alcoholic ale I've genuinely liked (I like it more than normal beer, to be perfectly honest), and is considered to be good for the health. Since real alcohol in iceland is so freaking expensive, if you like beer and don't mind not getting drunk, try some of that.
Before going, read the e-book 'Zombie Iceland'. It's a reasonable zombie story wrapped up with tonnes and tonnes of random information about Iceland. Quite short, but I'd say worth the €10...
This is probably slightly irrelevant now, but when I was there I passed up the chance to buy a traditional icelandic jumper, thinking I'd be mocked if I wore it at home, and they immediately became top indie fashion where I live. Bummer.
Hitchhiking is do-able in Iceland - it's safe, and relatively high numbers of people do it (though maybe not in March).
Get your Icelandic currency in Iceland, from an Icelandic bank. They're desperate for foreign cash since the Ice Save fiasco, so you get rates as good as the best bank-transfer-to-currency services in the UK.
Hope I've helped a bit.