Neither of those, just: I wish you a really nice day Sentences like I wish I were ... I wish I had ... etc indicate you are sorry for not being/not having

Happy birTHday buddy. Have a really nice day! If you still want you use the verb WISH in the sentence, you could say. I wish you a wonderful birthday and a great day!

The way you're trying to put it should be more like: Happy Birthday buddy. I hope that you have a very nice day. If you want to use "wish" in the sentence, then you could put: I want to wish you a Happy Birthday and a wonderful day. I hope that helps! :)