Turning 14 isn't considered a milestone, although it's still an important birthday. By her 14th birthday, your teen is probably more comfortable in her role as a teen, and trying more and more to be an adult. Avoid planning an outlandishly childish birthday celebration for her, or you'll run the risk of embarrassing her (unless she requests such as party, of course). Give her some freedom to recognize her new maturity.
Plan a grown-up inspired party for your teen to act grown up with his friends for a day. This could include a casino theme, nightclub theme or Hollywood-style party complete with paparazzi. Serve mixed drinks (sans the alcohol, of course) and set up casino games or other entertainment for the teens. Host the party at home, at a nightclub or another after-hours spot.
Plan a dress-up party with your teen, if she enjoys dressing up. The party can consist of a formal masquerade, scary costumes or even a specific theme such as pirates, aliens or the Renaissance period. Request on the invitations that guests dress in your particular theme, and decorate your party space to transport your guests to wherever you want them to be.
Outings can help your teen feel older and more mature, if he's given the freedom to venture out on his own, within reason. Take your teen and a few of his closest friends to a nice restaurant and let them sit by themselves. Plan a movie outing, allowing the teens to see a different movie than you. Let the teens go to a theme park, concert or sports event by themselves. Designate a spot for them to meet you, and have your teen text you at certain intervals to check in.
Spa parties, scrapbooking, ceramics and beading are some activities that can be enjoyed in your home or at professional establishments. Pack in a few different activities to keep the teens occupied all night for a sleepover. Go-kart racing, paintball wars and camping out are activities that might interest your teen, as well. Plan her birthday celebration around something that she enjoys and she'll have fun for her 14th birthday.
Backyard barbecues, pool parties, luaus, scavenger hunts, reality TV shows or almost anything you can think of could serve as a theme for the party. The teens could each put on a show based on your theme, from musical numbers, plays or even a runway show. Encourage the teens to work together and utilize each other's strengths.