Can you do Jökulsárlón as a day trip from Reykjavík?+
You can, and it is a fourteen-hour day (/experience/150518/) with about an hour at the lagoon and short stops on the South Coast. It works if you have one day and must see icebergs. It does not work well if you also want the South Coast properly. The honest answer is a two- or three-day tour (/experience/6131/, /experience/7358/) or a self-drive with a night near Höfn or Skaftafell.
What is Diamond Beach?+
The black sand beach at Breiðamerkursandur where icebergs from Jökulsárlón wash out to sea and back onto the shore. It is across the road from the lagoon car park, a five-minute walk. Iceberg size and number vary daily with the tide and season; some days it is scattered ice, others it is boulders. Stay off the ice and away from the waterline; the waves here behave like Reynisfjara. Morning light is best.
When are the ice caves open near Jökulsárlón?+
Roughly November to March. The natural blue caves under Vatnajökull need sustained cold to be safe, and operators like Arctic Adventures (/experience/17337/) open and close them as conditions allow. The location changes every year. Outside those months there are no natural blue caves here; multi-day tours advertised with an ice cave in summer usually mean the Katla cave near Vík instead. Check the tour page for which cave and which dates.
What is there to do in Skaftafell?+
Glacier hikes on Falljökull, the accessible tongue of Vatnajökull, from short introductions (/experience/17340/) to longer treks (/experience/1214280/), and the walk to Svartifoss, a waterfall framed by basalt columns, about an hour and a half round trip. Skaftafell has a visitor centre, campsite and cafe, and it is a good base for Jökulsárlón, forty minutes east. Fjallsárlón, a quieter glacier lagoon with Zodiac trips (/experience/1248016/), is on the way.
Is Jökulsárlón worth it in summer without an ice cave?+
Yes. Summer is when the boat tours run on the lagoon, and a Zodiac among the icebergs (/experience/716547/) is arguably better than a cave. The glacier hikes are open, the days are long enough to see the lagoon at midnight and Diamond Beach is the same. What you lose is the blue cave. If that is the one thing you want, come between November and March and accept shorter days.