It is true that the accusative case underlies each of these examples, but, in fact, only the first one uses this case without modifying it. Numbers два, три and четыре will override words in the nominative and accusative cases, making the new case genitive singular. Further, numbers пять through девять will cause a change from nominative/accusative to genitive plural.

Thus:
На одну поездку: Accusative singular.
На две поездки: Looks plural, but is technically genitive singular.
На пять поездок: Genitive plural.