I would suggest housing beers all day and night (PBR tallboys, Keystone, or Natty Ices would be best) up until the clock strikes midnight. Top each hour off with a shot of bourbon with everyone. If you start at 2 or 3 you'll be blackout come midnight. This would be ideal, then hit the bars at midnight, make sure the friends have a camera and let them buy you shots all night. The closer should be a microwaved shot of milk and tabasco sauce to make you bow to the alcohol Gods.
The enjoyment of legal alcohol on the 21st, IMO, should be the actual night of your birthday, at a bar/restaurant for dinner.
Granted I am still a ways from the 21st, but no rookie to celebrating 21st's. Celebrate the fact you wont have to use a fake ID anymore.