The ferry is for people only, not cars. However, there are four peninsula areas accessible by automobile: Deer Island [1], Nut Island [2], Worlds End [3], and Webb Memorial [4]. These areas offer exciting opportunities for visitors who want to combine the convenience of determining their own schedule with a great, fun experience.
Links:
[1] http://www.bostonharborislands.org/deer
[2] http://www.bostonharborislands.org/nut
[3] http://www.bostonharborislands.org/worlds-end
[4] http://www.bostonharborislands.org/webb-memorial
[5] http://www.bostonharborislands.org/mainland-deer-island
[6] http://www.bostonharborislands.org/mainland-webb-memorial-park
[7] http://www.bostonharborislands.org/mainland-worlds-end
[8] http://www.bostonharborislands.org/mainland-nut-island