는 거 재밌다

doing X is fun

-는 거 turns a verb into 'the doing of it' and 재밌다 says it is enjoyable. It covers both watching something (형 반칙하는 거 재밌으니까, 'him cheating is funny') and doing it yourself (과일 사고 이러는 거 재밌어, 'I enjoy buying fruit and stuff'). Every line here is casual speech: 거 is the spoken contraction of 것, and 재밌다 the contraction of 재미있다.

Register: casual spoken