So you're essentially asking the difference between these two sentences:
- (1) 店長が私に、二十歳のお祝いにカメラをプレゼントしてくれた。
- (2) 店長が私に、二十歳のお祝いにとカメラをプレゼントしてくれた。
Here, the に after お祝い is a role/function marker ("as" or "for" in English). It's the same に as in お土産に切手を買う, 彼を助手に雇う, お詫びにランチを奢る, etc. This に is not a place marker because, 1) to mark the place where some action takes place, you have to use で, not に, and, 2) while お祝い can refer to a formal event, we usually say お祝い会 or お祝いの席 for that purpose. (In addition, did this character really have a party and invite the shop manager? That's not something Japanese people commonly do...) It's not a time marker because 二十歳のお祝い does not represent time (while 誕生日 does).
This と is a quotative-と. That is, Sentence (2) means 二十歳のお祝いに was something actually said by the 店長. So (2) is basically the same as the following sentence:
- (3) 店長が私に「二十歳のお祝いに」と(言って)、カメラをプレゼントしてくれた。
Saying "(I'll give this to you) as the celebration", the manager gave me a camera.
The omitted verb after と is not 思って but 言って because this happened when he actually gave the camera. Of course you usually do not have to translate the sentence verbosely like (3). Although Sentences (1) and (2) are slightly different in Japanese, they may be translated into English the same way.