The end of the First Age brought about the drowning of Beleriand, while the Second Age closed with the Fall of Numenor. Nevertheless, both of these events left behind islands - each with its own distinct character.
Closest to Middle-earth, Himling was once Himring, Maedhros' hilltop fort. For centuries, it guarded the March of Maedhros, until at last it fell in the tumultuous Nirnaeth Arnoediad. Despite Morgoth's destructive madness, though, we can assume that the fortress remained - broken, perhaps, and breached, but still intact enough to offer a home for Orcs and other such creatures.
The War of Wrath, taking place on the barren Anfauglith, would not have reached Himring: the forces of the Valar arrived from the west, while Himring lay east of the battlefield. It would have been drained of its garrison, but not attacked - and if it had become a true Orc warren, it may still have been inhabited by any who were unable to go to battle.
We do not know the precise cause of the sinking of Beleriand, but we can assume it was violent. Earthquakes would have shaken Himring, and waves roared around and perhaps even over it. In the end, though, it stood alone, an island in the new ocean - and crowned with the last surviving fortress of the Noldor of the First Age.
And as time passed, the surviving Orc population would have bred, and multiplied, until the island became a seething pit of them: servants of Morgoth in the house of Maedhros.
By far the largest surviving island is Tol Fuin - formerly Taur-nu-Fuin, formerly Dorthonion. The largest mountain range in Beleriand west of the Ered Luin, Dorthonion would have easily survived the drowning. Indeed, its slopes were likely still graced with the pine trees beloved by Beren and Treebeard alike. With the fall of Angband, the Nightshade could even have lifted a little - but not much. Tol Fuin still means 'the Isle of Nightshade', and it would be a natural place of retreat for fleeing Orcs.
If a traveller from Third Age Middle-earth were to brave the Orc hosts (perhaps by only landing during daylight), what would they find? The homes of Beren's people are unlikely to have survived Sauron's assault, or the following six thousand years, even if Ladros - the lowest stretch of Dorthonion - remained above the waterline. Tarn Aeluin, hallowed by Melian, was nestled in the mountains above Ladros, and would still be visible - so the site of Barahir's death could be visited.
The Ered Gorgoroth still stood proud above the water - and were probably still infested with giant spiders. Rivil's Well, where Beren reclaimed the Ring of Barahir, seems fairly low-lying, and may well have sunk. And of course, the highest mountains of the Dorthonion range lie at the western end of the chain - the Crissaegrim, or Encircling Mountains of Gondolin.
Could Gondolin itself have survived the flooding, and still be standing there in hidden Tumladen? It's difficult to say. The waters shouldn't have risen high enough to flood through Cirith Thoronath, and the underground river may well have been blocked - but six thousand years is a long time. Even if the sea never broke through the Encircling Mountains, there was no natural outflow for rainwater. Surrounded by its high mountains, broken Gondolin may stand at the centre of a circular lake - or, indeed, may be underwater, clearly visible beneath crystal waters which no breeze can stir.
The strangest and most unnatural island is Tol Morwen - the mound where Turin slew Glaurung, where Nienor took her own life, where Turin fell on his black sword, and where Morwen Eledhwen finally laid herself to rest. The Stone of the Hapless, recording their fates, still stands, looking out over the empty sea.
Except it really shouldn't. There are countless hills and mountains in Beleriand taller than Tol Morwen. If natural processes had led to its survival, then Amon Rudh, Amon Ereb, and the Ered Wethrin ought to still be visible, too. Nevertheless, Tol Morwen stands alone - uplifted, it seems, by the power of the Valar themselves. Perhaps the disquieting prophecy of Turin returning at the Dagor Dagorath to slay Morgoth has more truth in it than we might think...
The only island to survive the wreck of Numenor, the Meneltarma was formerly the holy mountain, the Pillar of Heaven. It is said to have risen again after Numenor sank - and with it, one of the only known Hallows of Eru would have been restored to the world. After Taniquetil itself, the Meneltarma was the most sacred mountain in Arda.
Legend tells that, from the resurfaced mountain's summit, a traveller could see the Blessed Realm - but was it even possible to ascend the steep slopes? Did the island even exist, or was it just a rumour? And if one reached the top, what would remain of the ancient Hallow, which Tar-Miriel tried to reach even as the world was changed and Numenor broken?