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Let's pitch the tent while it's still light out.
Literal
Bright inside [at-に] tent [object-を] stretch [completion-しまおう].
内 ('inside/within') combined with an adjective and に creates a 'while still X' temporal clause: 明るい内に = 'while it's still light.' テントを張る ('to stretch/pitch a tent') is the collocation. 張ってしまおう = 'let's finish pitching' — the completion auxiliary ~てしまう + volitional. A compact outdoor-camping sentence.