For example, if I wanted to say "They used to travel every day", which would I use:
Ellos viajaron cada día.
Ellos viajaban cada día.
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That is the correct tense to use whenever you have an habitual action in the past.
As César mentioned, a possible literal translation of your example would be
Ellos solían viajar todos los días
Whether you use
cada día vs.
todos los días will depend on how the sentence continues and what is the main point you want to stress (if it's the traveling, the fact that it was done every day, or something else that happened every day that caused them to travel)
The imperfect tense is what you're looking for.
Viajaban cada día. They used to travel every day / They were traveling every day
or you can use the verb soler in the imperfect + infinitive, like so:
Solian viajar cada día. They used to travel every day.
The Preterite can be used like so:
Viajaron el verano pasado. They traveled last summer.