"In the summer" is generally more natural than "in summer" for this usage. I can think of situations where you'd want the latter instead, but they're pretty rare.
NB I'm only familiar with English - it might be different in American English.
In my dialect of American English, this is also more common. I've reported this issue on many exercises.
You could say куди as well, that would mean sort of "to where" (wohin) rather than "where" (wo)
I think that "Where did you travel" is incomplete. "Where did you go"( in the summer) sounds more natural. "Where did you travel To"(say - Ukraine) is also more natural but I accept that куди would have been necessary. Also more natural is "I travelled In" (say - Ukraine)