Q: I'm not talking about those big birthdays, like 18 or 21, just a regular birthday.
A: Spend time with family. Open their presents. Play some sport while wearing one of their favourite presents. A new T-Shirt. Use their presents as soon as they get them. Have a couple of social drinks with their friends and family. Add a bbq. Open their brand new notebook and hack the planet lol or make the best website ever. Or program the best hack proof secure os or database. Start a project with one of their new presents. Go bowling with friends. Visit their girlfriend and go out for dinner {spend time with the girlfriend} and if the don't have one just a group of friend's or best friends. Be antisocial and park at the beach and watch the sunrise. Plant a tree or a potplant that they received as a present. Help their Mum and Dad with the dishes after everyone has made a mess if they live at home. Visit friends. A Regular birthday :- watch a movie with some friends, cook a roast and play a fun game of sport. Maybe battle it out in a racing game on their new console :-)