kupanje

1. (in a tub or shower) bathing; bath
   Posle kupanja beba odmah zaspi. — After her bath the baby falls asleep right away.
2. (in the sea, river, pool) swimming; bathing
   Kupanje u Dunavu ovde nije dozvoljeno. — Swimming in the Danube is not allowed here.
   Idemo na kupanje na Adu? — Shall we go swimming at Ada?

"Ići na kupanje" means to go swimming.