。
It's fun to play football after school.
Literal
After-school [at-に] football [object-を] do [nominalizer-の] [topic-は] fun
The nominalized clause 放課後にフットボールをするの ('the act of playing football after school') becomes the topic via は, and 楽しい is the comment. A clean example of のは...だ structure used to make a judgment about an activity.