Last Saturday, my father and I ... to a small fishing village.

Explanation: This sentence describes a past event. For past events, we use the Past Simple tense. The structure for affirmative sentences in Past Simple is Subject + Verb (past tense). When we use the auxiliary verb 'did' for negation or questions, the main verb remains in its base form. However, in affirmative sentences, we use the past tense form of the verb directly. Among the options, 'walked' is the correct past tense form of the verb 'to walk'. 'Didn't walked' is grammatically incorrect because after 'didn't', the base form 'walk' should be used. 'Did walk' is used in questions (Did you walk?) or for emphasis in affirmative sentences, but 'walked' is the standard affirmative past tense form.