Do you have a pool, a lake, the beach - great places to hang out and have a low key birthday. If an option, you could do a sunset cruise.
Play some music and serve some food, rent some movies.
Go to a camp site, build a fire and bring food, make smores.
Rent a place at a park that has a barbeque or just bring food and drinks to the park.
Consider what you normally enjoy doing with your friends for the fun part.
If you and your friends are between 12-16, you can do a spa party and give each other manicures and facials. You might have to buy the facial cream, nail polish remover, cotton pads and emery boards in the drug store but you can ask your friends to bring their nail polish etc. You can watch a movie. For food, order in pizza and buy some soda. For dessert you can have a birthday ice cream cake or any birthday cake or cupcakes.
If you can keep your guest list down to 10, you could all make your own English Muffin or Boboli (ready made pizza crust) Pizzas. Just put out the toppings and everyone can make their own pizza. Instead of a birthday cake, your friends can make their own ice cream Sundaes. Just put out 2 or 3 flavors of ice cream and some topping like chocolate syrup or hot fudge, caramel topping, nuts and cool whip or whipped cream.
Backyard party. Music, good food and fun games.
It's my birthday soon I want to have a party with10-20 people low-key like we could chill.but what would you do for a low-key party where you can still have a lot of fun.