If the weather's good tomorrow, let's go hiking.

Literal

Tomorrow weather [subject-が] good-if, hiking [purpose-に] go-[let's-ましょう].

~ましょう is the polite volitional — 'let's do X.' Combined with the ~ば conditional, it creates a classic conditional-invitation pattern. ハイキング is a katakana loanword directly from English.