Outdoor

Outdoor (WITHOUT s) is an adjective. Outdoor = located, suited for, or taking place in the open air: ‘He enjoys the outdoor life.’ ‘There are many outdoor activities in and around Coimbra.’

Outdoors (WITH s) is an adverb. Outdoors = outside a building: ‘He likes to work outdoors.’

Outside = when you mean ‘away from’, use (get/go) out of (NOT outside): ‘It does you good to get out of the city now and again.’ ‘Without a passport, he would never have been able to get out of the country.’

In British English, outside is not used with of: ‘You aren’t allowed to park outside the bank.’

In American English, both outside and outside of are used.