。
Building model spaceships is fun.
Literal
Model [possessive-の] spaceship [object-を] make [nominalizer-の] [topic-は] fun is.
の-nominalization with 面白い as the predicate. 面白い ('interesting, fun, amusing') is one of the most over-worked Japanese adjectives, covering a broad range of positive reactions. 宇宙船 ('spaceship') is a Sino-Japanese compound of 宇宙 ('space, universe') + 船 ('ship').