I often spend my holidays outdoors.
Eating outdoors feels great.
戸外の気温は氷点下だった。
The outside temperature was below freezing.
天気がいいので戸外に出てみよう。
Since the weather's nice, let's go outside.
It's good to get some fresh outdoor air once in a while.
野外 suggests open fields or nature, often used for outdoor events like 野外コンサート, whereas 戸外 simply means the space outside a building.
外 is the common, casual word for 'outside', while 戸外 is more formal and typically appears in writing.
Compound of 戸 (と, 'door') and 外 (がい, 'outside'), literally 'outside the door'.