Reframe the question
Most travel sites answer "what is there to do for free on KI" with a list of things you would pay for anywhere else. We can do better than that. Kangaroo Island is mostly nature, which means most of what is worth doing is either free outright or pay-once-and-stay-free. The beaches, the dunes, the back-road drives, the lookouts, the dolphin pod off Penneshaw, the farmers market in the cool season, the free honey tasting at Island Beehive. None of those have a ticket gate.
The list below is the locals' shortlist. Things we send friends and family to first when they have a day to fill and a fuel tank to spend.
Walk Vivonne Bay beach
Vivonne Bay is one of the best beaches in Australia, regularly ranked in the national top ten, and it is free to walk. Park at the jetty at the end of Jetty Road, walk west along the white sand and you have around 4 kilometres of beach in front of you. The water is the protected side of the south coast, so the surf is gentler and the bay is one of the few south-coast swim spots that families can manage with kids. There is no entry fee, no parking fee, no shop, no kiosk. Pack a towel and lunch.
Walk the Little Sahara dunes
Visiting the dunes at Little Sahara Adventure Centre is free. You only pay if you decide to hire a sandboard or join a guided tour. The main dune is around 70 metres, and a 10-minute climb up the back side gets you the view: gypsum sand under your feet, the south coast in front, the Southern Ocean on a clear day, and bushland behind. Bring covered shoes. The sand can get hot underfoot in summer.
If you want to slide back down rather than walk, the on-site hire shed runs Sandboard and Toboggan Hire at $37 per board for up to 3 hours, plus $10 helmet hire. All ages, walk-up at the shed, no booking required. That is optional. The dunes themselves are free to explore.
Watch the dolphins from Penneshaw jetty
A local pod passes the Penneshaw foreshore most mornings, usually between 8 and 11 am. Walk down to the jetty with a coffee, scan the water, and you will often see them inside ten minutes. No tour required, no boat required, no fee. Some days they come close enough to the jetty that you can hear them. The pod is wild and movements vary day to day, so there is no guarantee, but the success rate over a week is high. The benches at the foreshore are the comfortable seats for it.
Stokes Bay rock tunnel and swim
Stokes Bay on the north coast looks like a closed rocky cove on first arrival. Walk to the western end of the visible beach, find the gap between the boulders, and the tunnel through the rocks opens onto a protected swimming pool inside. The pool is shallow, the water is calmer than the open coast, and it is one of the better family swims on the island. Free. Drive in via Stokes Bay Road, park near the visible beach, and look for the tunnel marked by the path through the boulders.
Cygnet River self-guided drive
The back roads through Cygnet River farmland are some of the best wildlife driving on the island, especially at dawn and dusk. Kangaroos in the paddocks, koalas in the roadside gums, and the occasional echidna crossing the road. Drive slowly, stop in safe lay-bys, and bring binoculars. The loop from Kingscote west via Cygnet River Road and back via the Playford Highway is about 90 minutes including stops. Free, no permit, no entry fee.
Penneshaw farmers market
The Penneshaw farmers market runs the first and third Saturday morning of each month, on the foreshore at Penneshaw. Local growers, bakers, honey producers, oils, jams and a coffee cart. Free to browse, you only spend if you want to. It is the easiest way to meet the people who make the food on the island and to take some of it home. Park along the foreshore, allow 45 minutes.
Free honey tasting at Island Beehive
Island Beehive (the visitor-facing brand of The Kangaroo Island Ligurian Bee Co) at 59 Playford Highway, Kingscote, runs a free honey tasting at the shop counter. That is separate from the paid 30-minute Behind The Scenes Tour, which is a different product. The tasting is open during shop hours, no booking required, and you can try the season's pure Ligurian honey range before you decide what to take home. KI is home to the world's last remaining pure-bred population of Ligurian bees, protected by quarantine since 1885, which the staff are happy to talk about at the counter for free.
Watch sunset at Remarkable Rocks
Remarkable Rocks sits inside Flinders Chase National Park on the south-west corner of the island, so the park does require a vehicle day pass. The sunset itself does not cost anything extra. If you are already in Flinders Chase on your park pass, stay until the last hour of light. The wind-sculpted granite domes turn orange in the late sun, the wind drops, and the south-coast cliffs catch the colour. Time it for 30 minutes before official sunset and you will have the rocks largely to yourself, because the day-trip crowd is usually gone by then.
Walk a section of the Heysen Trail
The Heysen Trail crosses Kangaroo Island as part of its 1,200 kilometre run through South Australia. Multiple free trailheads dot the island, the most accessible being the Antechamber Bay end on the east and the Cape Borda section on the far west. You do not need to walk a multi-day section to enjoy it. A two-hour return walk from any of the marked trailheads is a free way to spend a morning, with coastal and bushland scenery the whole way. Trail maps are available free at the Penneshaw and Kingscote visitor centres.
The free list in one paragraph
Walk Vivonne Bay. Climb the Little Sahara dune. Watch the dolphin pod off Penneshaw. Swim at Stokes Bay through the rock tunnel. Drive the Cygnet River back roads at dawn or dusk. Browse the Penneshaw farmers market on the first or third Saturday. Taste honey for free at Island Beehive. Watch the sunset from Remarkable Rocks if you are already in the park. Walk a Heysen Trail section. That is more than a day, more than a weekend, all of it free or close to it.