Is this a decent present for my boyfriend's 21st birthday?

Sounds like a good idea on paper, but not necessarily a practical idea. I mean is he really going to eat 21 cookies? And what is he going to do with 21 balloons? If you are limited with money then don't throw it away. I think your flask idea is a great idea which as a man he would probably appreciate a lot more than cookies and balloons. Maybe you could make a CD of all the songs that will remind him of his 21 years. Maybe a song from each year he has been alive - even every no.1 in the charts up to his birthday!