。
Making model airplanes is fun.
Literal
Model airplane [object-を] make [nominalizer-の] [topic-は] fun is.
Classic の-nominalization frame — [clause] + の + は + [predicate]. 模型飛行機 ('model airplane') is a transparent compound. Good minimal example of the 'X-ing is Y' structure in Japanese: nominalize the action with の, mark it with は, predicate with a simple adjective/noun.