You can visit Kangaroo island on a one day tour from Adelaide although it will be a long day. The ferry ride (45 minutes) departs from Cape Jarvis which is just over 100 klm from Adelaide so allowing time to board and exit the ferry the one way trip from Adelaide to Penneshaw on Kangaroo Island is a little over 3 hours.
If you have the time then the best way to experience all that Kangaroo Island has to offer is to spend a couple of days there. This gives ample time to see all the natural attractions the island has, with time to sample the local produce the island is famous for.
Flinders Chase National Park is the largest National Park on the island and include such icons as Remarkable Rocks and Admirals Arch as well as light houses and walking tracks. There is the Seal Bay Conservation Park which features a colony of Australian Sea Lions and Hanson Bay Wildlife Sanctuary's Koala Walk. While traveling around the island you are likely to see the endemic Kangaroos which gave the island its name, echindnas and a wide array of bird life.
For a culinary delight sample wines from one of the dozen wineries on the island, their world famous honeys, or the world class seafood from around the island.
If it is an action holiday you are seeking Kangaroo Island has that covered, you can go surfing at Vivonne Bay, ride the sand dunes of Little Sahara or head out on one of the many walking and hiking trails.