mine sucked too, spent it on the couch at my parents house sick as a dog with a virus. spewed about 15 times and i didn't even have the fun of the night before to blame it on.
my 20th and 22nd were fuckin ace though, partied HARD for a couple of days with my mates, lots of drugs and booze and spliffs.
a lot of my mates have done stuff like going out for a fancy dinner then clubbing for their 21sts and some people's parents threw big parties with loads of family and friends at them, free bar free food etc in like a marquee or hire a club out etc. i personally prefer just going out to the local with the best mates and then hitting a few clubs before bringing the party home. that way you don't need to worry about any shit and everyone else is buying your drinks!