My only advice is to go all out!! my boyfriend will be 21 next month and I'm getting a room and then a girl that I work with is a massage therapist and is going to come with another massage therapist and give me and my boyfriend a massage in our room. I plan on having white candles lit everywhere with some relaxing music. Then I will go to his favorite resturant and get carry out so that we can eat in our beautiful room. Then we are going to get ready and he's going to go to a titty bar while me and my friends (who will also be celebrating her b-day) will go to the club while the boy have a night to them selves. Then we are going to meet back at the hotel and get in the hottub.
Thats my plan. Also I'm making a cake that looks like a nakes woman and for the croutch area I'm going to use coconut. I think that its is a good idea to have an alone celebration but also let him go out and have his fun like drink or go to a 21 and over bar. Thats what everyone waits for when they turn 21 to drink or go clubbing. Try to imagine what you would like to do but also include what he likes or dislikes.. Keep it original and have fun with it I mean your only 21 once!!