I have trouble understanding when to use concrete and abstract verbs like їздити and їхати. For this, I would have expected їхати because it was multidirectional, i.e. if they went somewhere during the summer they obviously came back afterwards. Is їздити used here because it's a single, completed action? Would it be incorrect to use їхати instead?
Firstly, їхати is not multi-directional. Куди їхати? - Їхати кудись. To ride somewhere.
Secondly, I have a hard time trying to explain the verb choice to you as well :)
I would put it like this.
"Ви їхали куди-небудь влітку?" sounds strange and IMHO is wrong. It somehow has a feeling as if you were going somewhere but didn't get there, didn't finish going. Well, їхати is an unfinished aspect, but it doesn't always mean the action is not finished.
"Ви їздили куди-небудь влітку?" is the best choice. Implies travelling around in many directions, visiting without any clear purpose, not just a trip but a journey
"Ви поїхали куди-небудь влітку?" is correct. It's in the finished aspect. It's not preferable to the previous one for the reason that it sound as if you either 1) went only to one place (so the question sounds less weird if you do know that the person who's asked planned to go only to one place); or 2) went somewhere and didn't come back O_o
Man I'm getting so confused by the differences between concrete and abstract verbs and perfective and imperfective aspects. Also, check the usage notes of https://en.wiktionary.org/wiki/%D1%97%D1%85%D0%B0%D1%82%D0%B8 and https://en.wiktionary.org/wiki/%D1%97%D0%B7%D0%B4%D0%B8%D1%82%D0%B8
The information is opposite in each, I think I read the incorrect one.
So let me see if I have this straight now: їхати is concrete (unidirectional, a definitely directed motion, or a single, completed action) and imperfective (habitual, unfinished, continuous, repetitive or in progress).
Їздити is abstract (multidirectional or indirect, or whose action is repeated or in a series) and imperfective.
Поїхати is concrete and perfective (completed action).
Do these definitions sound right to you?