Do you remember in the movie Charlie and the Chocolate Factory when the chocolate factory reopens and the grandfather is explaining how overnight someone had put up signs throughout the town in search for the chocolate bars that had the golden tickets, and then it went to the scene where many mini-buses were leaving the factory in many directions to deliver the chocolate bars… those white buses darting in every direction was the feeling when waiting for the excursions in Reykjavik.
For those considering a visit to Reykjavik the main tourist companies are (1) Reykjavik Excursions and (2) Reykjavik Sightseeing. Both companies pick up/drop off at your downtown hotel, and within 15 minutes of the assigned time, are relatively fair in pricing. For a more convenient purpose, choose Reykjavik Excursions, this is the most popular excursion company that has each of the mini-vans picking up/off from the hotel and then transporting you into a bigger bus to the BSI terminal. For a more informative tour, I would recommend Reykjavik Sightseeing. Their guides are very knowledgeable, and typically a local in the area. Additionally, Reykjavik Sightseeing allows you to book Northern Light night tours while you are on your day tour.
I would recommend if you are planning a trip to spend your first two days on the hop on/off bus, with Sightseeing Tour Bus. This allows you to get acclimated to the downtown area, with the bus stopping at each tourist attraction; with another bus to come take you to your destined next step within the next 30 minutes. Plus a cheap way of getting transportation, the city is really small.
Recommended excursions/tours: Golden Circle Tour with Reykjavik Excursions, Northern Light Tour booked with Reykjavik Sightseeing Tour, South Coast Tour & Snowmobiling with Reykjavik Sightseeing Tour, and Blue Lagoon.
*If you are going to see the Northern Lights book this the first night, because in the event the weather conditions are not good, you can rebook free of charge for the next night, and then the next and next until you see them. Separate post to come!