Whatever it is don't go to a bar and get drunk.Find a place you always wanted to see and go their on a weekend. Forget the big parties you can always have those . Sugar make it nice and cozy. What's a better gift then you.?

i disagree with the first answer but maybe that's just because i am 21 myself!! i don't have any great answer for u except to let u know what i did for my boyfriend's 21st. his 21st birthday was in may of 06 .. he went to six flags with two of his best guy friends ( i didn't go bc i'm afraid of rollercoasters and that wouldve ruined the day to have me dragging along going on no rides!) but i did pack them a cooler with beer, sodas, water, snacks, sandwiches, etc. which they really enjoyed. i used facebook (if u have a facebook account it's really easy to make an event and invite people) to make an event and i planned a surprise dinner. i made sure people knew that they would be paying for themselves (i was 19 .. no way i could foot the whole bill! people understand that..) he didn't know about it but his friends did so they both made sure that he was back in time and casually said "hey, let's get back around 7 and go grab some sushi" (his favorite food is sushi -- i don't eat it but hey, it's his day!). when he got to the restaurant there were about 15-20 of us there. there was no corny "surprise!" and i'm sure he knew something was up, but he was grateful to be with all of his friends, eating his favorite food, etc. we did saki bombs and had a great night. afterward a bunch of the people came back to his apt and hung out including dinner guests and others who couldn't make it to dinner. so bottom line is just do something that would make him happy, something that he enjoys!! drink all he wants it's his 21st!!

maybe you should get him a nice puppy from the pet store, he'll love that

Get him an Apple iWatch