Give me a round-trip ticket to Newark, please.

What's the round-trip fare to Chicago?
シカゴまで往復いくらですか？

fly a solo round trip between
～間を単独で往復飛行する

Give me a round-trip ticket to Newark, please.
ニューアークへの往復切符を下さい。

I made a round trip to the station by car twice today.
今日は車で駅まで2往復しました。

I'd like three round-trip tickets to ~. Two adults and two children.
～まで往復、大人2枚子ども2枚。

I'd like three round-trip tickets to Chicago -- one adult and two children.
シカゴまで往復、大人1枚子ども2枚。

One-way or round-trip?
片道か往復のどちらですか？