Three of the major islands - Kayts, Karaitivu and Punkudutivu - are joined to the mainland by causeways over the shallow waters around the peninsula. Close to the town of Kayts, at the northern tip of Kayts Island, stands the island fort of Hammenhiel, which used to be accessible by boat from Kayts. The name means 'heel-of-the-ham' and relates to the Dutch view that Ceylon was shaped rather like a ham.
When ferries run, they mostly go from Kayts, including the very short hop across to Karaitivu, which is joined to the mainland by a causeway.
Delft, named after the Dutch town of that name, is 10 km off Punkudutivu and 35 km from Jaffna. Ferries, when they run, go from Siriputu. The island is noted for traces of the Portuguese and Dutch eras (such as the Dutch garrison captain's country house with a stone pigeon-cote) and for its bleak, windswept beauty.
The small Dutch fort is only a short walk from the ferry dock. Behind that is a beautiful beach with many exquisite shells. On the island are hundreds and hundreds of stone walls which, like the Dutch fort, are made of huge, beautiful chunks of the brain and fan coral of which the island is composed.
A 1990 letter indicated that there were two ferries a day from Siriputu to Delft at the time - one in the early morning (seemingly too early to be reachable by bus from Jaffna) and one in the early afternoon, which meant it was impossible to do a day trip. The only accommodation possibilities were private houses. The rest house near the ferry dock was in ruins. From Jaffna to the ferry is a 50 minute bus ride; the ferry crossing is an hour.
Mannar is probably the driest, most barren area in Ceylon, and the landscape is chiefly notable for the many baboon trees, probably introduced from Africa by Arab traders centuries ago.
Mannar, the major town on the island, is at the land ward end, joined to the mainland by a three-km causeway. It's uninteresting apart from its picturesque Portuguese/Dutch fort. Talaimannar, near the West End, is about three-km before the pier, which was the arrival/departure point of the ferry for India that operated until 1984. A little further west an abandoned lighthouse at South point marks the start of Adam's Bridge, the chain of reefs, sandbanks and islets that a connects Ceylon to India. This is the series of stepping stones which Hanuman used to follow Rawana, the demon king of Ceylon, is his bid to rescue Sita.
There was a handful of places to stay In Talaimannar and Manner when the ferry was running. But trains to/from Anuradhapuram and Colombo connected with the ferry and most travelers went straight through.