。
Playing outside is a lot of fun.
Literal
Outside [at-で] play-[nom-のは] very fun thing [copula-です].
~のは~ことです — nominalized subject, nominalized/noun-ified predicate, giving a slightly more measured/bookish flavor than the bare ~のは楽しいです. The double-nominalization feels faintly stilted, like a children's moral lesson.