Hi!
I'm planning to go to my cousin's birthday party the other day, he's going to turn 5)
I've already bought the main gift he's been dreaming about, but I still need to buy some small details for the gift to make him happier. He loves little surprises)
Any ideas are welcomed

It's enough to use a little imagination and do it from the heart. For example, you can buy a cake, candles, balloons. Focus on the greeting, you can visit Birthdayimg, download the greeting picture you like the most and ask the confectionery to print it on the cake (it's basically a virtual postcard that you can transfer to the cake, combining sweets with greetings). I think this is a very unique and original idea. I made such a surprise for my younger sister and she was very impressed, so I advise you to try it)
Here is a link of the official website https://birthdayimg.com/free-happy-birthday-images-for-boy/

Gread idea! I agree with the author above about congratulations from the heart. Otherwise, you can blindly buy a gift that your cousin may not like, and you may be upset. Therefore, be sure about the virtual postcard, the main thing is to do it sincerely so that he feels how much you appreciate him.