Iceland in May

You'll have no problems booking when you get there.

You shouldn't have trouble with the Golden Circle tour, unless there is a specific company you want to go with. In that case, it might be wise to book in advance, just to be sure. They all take the same route, but some are smaller (and in my mind, better) than the massive coach-based tours. The one I took had a smallish bus with space for about 20 people, and our driver/guide, Dooley, was delightful.

I would book the Blue Lagoon ahead of time. It's not likely to "sell out," but you don't want to get stuck with an inconvenient time (bear in mind that it takes about 45 minutes to get there, and back to Reykjavik, by bus).

As for the Reykjavik card, take a look at the things it discounts and then decide if they apply to you. It covers transport, but Reykjavik is small and compact. If you don't mind walking (including in the occasional rain and wind), I'd not bother. I do not believe it covers the bus from the airport, but you should doublecheck that.

Caroline

I spent 4 or 5 days in Iceland in May 2014. Personally, I don't know that the Reykjavik card is worth it unless you plan to use public transport a lot or there's something specific that you want to see that's covered by the card. Reykjavik is a small city and definitely walkable, especially if the weather is cooperative. When I was there, it was overcast or sunny most of the time, with temps in the high 50s.

For the Blue Lagoon, we didn't book anything ahead of time. In fact, we went there to kill time after we got stuck at the airport due to a strike. We just bought tickets from one of the tour companies' kiosks at the airport, and they covered bus fare + entry.

For the Golden Circle, we just rented a car and did the "tour" ourselves, so that's another way to do it if you like :)

We were there in April 2015, just before the Northern Lights season ended and just before Puffin Season started. We didn't get Reykjavik cards, and went to the Fontana pools/springs rather then the Blue Lagoon. We did reserve our Jokulsarlon/glacier hike tour with Icelandic Mountain Guides before we left, just to ensure we would be on the roster and the trip would actually be a "go" that time of year. We also prebooked our tickets for the "How to be Icelandic in 60 Minutes" show at Harpa, as they only had 2 shows the entire month of April, one the night of the day we arrived. For our Elding whale watching boat tour, however, we just showed up at their desk by the harbor that morning for the afternoon boat, but lots of people seemed to just have shown up 10 or 20 minutes before the afternoon boat sailed, and they wound up taking out 2 boats! They guarantee you will see at least one whale, or you get a voucher good for another free whale watching tour, good for 1 year.

April was just too early for visiting Heimaey island, with its volcano, lava-buried houses, and visiting puffins, but if you're there in May and have 5 days overall, I'd definitely make time for a trip to Heimaey one day. Also, don't miss having bitafiskur (dried fish jerky) Icelandic style (slathered with butter), and some local licorice.

And the bread...oh my god, that bread :)

How do you get to Heimaey? Do you remember how much the ferry cost? that seems like an interesting day trip. I will be in Reykjavik for 5 days.

Bryan - best to research thru the Lonely Planet Iceland guidebook, and/or online. As I recall from some of our research (which, again, didn't wind up applying for the time we were there), there are 2 different ferries from 2 different departure points, depending on the season. There are also flights, but you'll have to decide on the cost-versus-benefit of ferry and plane. One ferry takes much longer as I recall, and maybe wasn't cheaper, but leaves from closer to Reykjavik, so there are several options, depending on the time of year and other factors.. Then, you have to mind the departure and return times, if you're set on sleeping in Reykjavk every night, and not possibly spending a night on Heimay itself. There may even be an outfitter or tour company (or several?) that will shuttle you from Reykjavik to the ferry, and then return you to Reykjavik at the end of the day. Sorry to not have more details, and maybe things have even changed in the past year, so getting up to date specifics from the ferry company will ensure you have the latest, accurate info. Have a great trip.