Vulcan Point in the Philippines is the world's largest island within a lake (Main Crater Lake) that is situated on an island (Volcano Island, aka Taal Island) located in a lake (Lake Taal) within an island (Luzon). Seems like straight out of Inception movie :)
Lake Manitou in Canada is the largest lake on an island in a lake. This is 100+ sq kms in areas. So not a trivial one. This lake also has islands in it :)
http://www.geog.ucsb.edu/events/department-news/802/an-island-within-a-lake-on-an-island-within-a-lake-on-an-island/
http://www.elbruz.org/islands/Islands%20and%20Lakes.htm
Details on these in wiki:
http://en.wikipedia.org/wiki/Taal_Lake
http://en.wikipedia.org/wiki/Lake_Manitou