Snorkelling and Diving: the best time to dive would be April to October. During June you may be able to do a midnight sun dive. The water temperature and visibility are relatively stable across the year. Water temperature ranging from 4ºC all the way up to 10ºC.
Marine wildlife: the peak months for whale watching are June and July. In northern Iceland, you will have a wider window to enjoy visits from humpbacks, minkes, and dolphins, which ply the Atlantic from May to August; a few humpbacks even stick around until the end of the year. Blue whales pass through in the summer, too.
Birdwatching: the best time for a birdwatching trip is definitely late May to early June when all migrants have arrived and birds are very conspicuous, defending their territories. At this time of year there is also 24-hour daylight so the very keen can stay out round the clock. 12 million Atlantic Puffins nest on the cliffs and islands of the country and from May to September there is plenty of locations to get close sightings of these birds.
Hiking and other outdoor activities: the country is home to some of the most diverse landscapes in the world. Summer is the main hiking season in Iceland. During this time of year, the weather is pleasant and the valleys and forests are in full bloom. You can expect temperatures ranging between 10-20°C, with the highlands being the only exception. Even in the summer, the Highland region can reach near-freezing temperatures. The chances of encountering bad weather are lowest between June and August so this season is your best bet for exploring the Icelandic outdoors.
Northern lights: the best way to optimize your chances of seeing the northern lights in Iceland is by visiting from late September to late March, it is dark after 6 pm, and one enjoys maximum chances. In November, December, and January, the nights are the darkest, allowing you to start seeking the aurora borealis from mid-afternoon and continue until nearly noon the next day if you are so inclined.
Arctic foxes: Hornstrandir Nature Reserve, an isolated peninsula in the northwestern corner of Iceland, is the best place to spot Arctic Foxes. It is possible to see foxes in other parts of the country, but it is considered quite rare. Icelandic summer nights are the right time to see Arctic Foxes. During the daytime, they stay in and around their dens. But at night, the foxes are more active and visible. It should be noted that organised trips to Hornstrandir occur only in May, June, July and occasionally August; for “winter shoots”, where the white fur is crucial, further planning will have to be made in advance.