My 21st birthday was the best ever!
I went out with my friends to a small lounge in my city with a bunch of friends.. Or rather, the older ones had beers, the younger ones had Coke or Sprite, I had one of those clear fizzy alcoholic fruit-flavored drinks, because I don't like beer.
I reserved a few booths near the dance floor and everyone came out. I wore a dress and made a point of wearing full coverage underwear on the off chance I fell down or something, which was a good choice because a drunk man with the same birthday as me picked me up and flashed my underwear to the entire bar.
I also got a lot of free drinks which may or may not have been related.
Also 21 is kinda meh. But it seems to be this official age.
I tried to book an area there for my 21st and they forgot my booking three times. The only reason I found this out was because I had a terrible experience there prior and expected that level of service. Ended up canceling it because I couldn't be bothered dealing with them.
I ended up putting a tab on at a local bar/grill and booking a section out. It was great. It's very hipster but it's very central, which was my main priority and it has good drinks of any type.
If you are wanting a whole venue booked, a friend of mine had about 30-40 people at her 21st there, had great staff and it didn't blow her budget.
Hope you have an fabulous birthday!