露天風呂 vs. 溫泉

露天風呂 = rotenburo or rotemburo = open air bath

お風呂に入る = ofuro ni hairu = to take a bath

溫泉 = onsen = hot spring, spa

部屋で浴衣に着替えて、タオルとバスタオルを持ったら、旅館のフロントで露天風呂の入湯券を買う。
Heya de yukata (yokui) ni kigaete, taoru to basutaoru o motta ra, ryokan no furonto de rotemburo no nyuutou ken o kau.

After you have changed into your swimsuit/bathrobe at your room and taken the towel and bath towels, buy the entrance ticket to the open air bath at the front of the hotel.