The most basic way to say this is: "Thank you for the present." You can change it by adding one or more adjectives to describe the present: "Thank you for the _____________ present." examples: "Thank you for the nice present." "Thank you for the nice birthday present." "Thank you for the thoughtful present" "Thank you for the thoughtful birthday present" "Thank you for the special present" ..... Other variations you can use: "Thank you for the thoughtful present. I had a wonderful birthday!" "Your birthday present was so nice. Thank you for making my day!" There really isn't a wrong way to thank someone. I hope this helps! :)