「你對啲咩嘢有興趣㗎？」we will understand you're asking "what thing you're interest in?" Usually only answer one thing. 「你對咩嘢有興趣㗎？」we will think you're asking "what something you're interest in?", that is mean the question not only ask one thing. But usually, no 啲 in this sentence, is not effect the meaning.

咩嘢 and 乜嘢, in fact no difference, you can use either one. 啲, this word is quite tricky. Sometimes no difference to use it or not in a sentence, like yours. But sometimes you cannot skip it, e.g. 呢啲咩嘢嚟㗎？ if you skip 啲 in this phrase, then it cannot become a normal phrase.

啲 means “some” in Cantonese， it also works if you said 你對乜嘢有興趣㗎. However, 你對啲乜嘢有興趣㗎, makes a more soft tone and more smooth when it is used in Cantonese. I guess it's similar like "what things are you interested in " and " what kind of things are you interested in "

啲 is usually following verbs.. eg. 你喺度（做緊啲）咩吖？ 你想（食啲）咩吖？

啲 is used as adv. here , strengthen the verb 對