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As a rule, transitive verbs in Italian use the auxiliary avere (ho camminato, avrò camminato) when conjugating in any of the compound tenses. However, camminare & viaggiare are intransitive and seem to be in the category as other verbs of motion: scendere, andare, cadere, etc. that use essere as a helping verb.
Why are these verbs exceptions to the rule?
Are there other examples of verbs that are intransitive yet use avere as an auxiliary verb?
For intransitive verbs, unfortunately, the general rule is that «non esiste una regola che permetta di stabilire quale ausiliare debba essere usato con ciascun verbo» («there is no rule that allows to know which auxiliary verb is to be used with each verb»). There are very vague rules: you can see something about it and about a tentative 3-part classification in transitive, intransitive, “inaccusative” verbs here, where the quotation above is taken from. However, there are lots of intransitive verbs to be used with avere as their auxiliary verb: for instance, apart from the ones you mentioned, esagerare, navigare, riposare, sbandare, marciare, sorridere...
