short trip; excursion; brief journey
A short trip or excursion, often for leisure or a brief getaway. Can be used for day trips or overnight stays.
週末に小旅行に出かける予定です。
I plan to go on a short trip this weekend.
彼女は友人と箱根へ小旅行に行った。
She went on an excursion to Hakone with her friends.
小旅行 is a general short trip that may or may not involve an overnight stay, whereas 日帰り specifically means returning on the same day.
Compound of 小 (small, short) and 旅行 (travel, trip). The exact historical derivation is uncertain, but the word transparently means 'small trip'.