Have a nice dinner with a couple friends then go to a hookah lounge & relax maybe do something fun like amusement parks or water parks drive somewhere far with a few friends take a couple shots (even though your 20) enjoy the fresh air go swimming bowling shopping etc spend time with family or just sleep & ignore everyone that's something I'd probably do :)
There are a couple of things that you can do for your 20th birthday. You may decide to throw a party and celebrate this day with your friends and family. A formal dinner with your friends and family can also be a great idea. Other fun activities for this day may include ice skating or watching movies.
You should go out to the club with your friends and have a few drink and a nice time. Or you can go out with your boyfriend if you have one. Or you can go to the movies and out to eat and maybe even go bowling. Or you can have some me time and go shopping.