「お誕生日には何をされたんですか」While the answer to this like likely very simple, why is される being used in this case rather than する? What is it implying grammatically? The Potential Form? Is it an indication of added politeness? It's clearly not Passive as an を is being used. What prompts this use of される?
「される」 is the honorific form of 「する」. The speaker/writer is showing some respect to the listener/reader here.
Another form of 「される」 is 「なさる」.
The sentence in question, under normal circumstances, should be in the active voice -- "What did you do on/for your birthday?".
If you want to discuss "pure" grammar, however, the sentence can be taken to be in the passive voice -- "What was done to you (by someone) on your birthday?". 「を」 is still needed for the passive voice.
Which one it was meant to be, only the listener/reader knows (but easily). That is why Japanese is an incredibly contextual language.