A 25th birthday party is a momentous occasion, and in Los Angeles, there's no shortage of things to do that are fun, but decide first if you want grown-up fun, or child-like fun. It's a quarter century, and worth marking with friends, doing something fun.
One way to mark the day is to go ride roller coasters, and nothing beats Six Flags Magic Mountain in Valencia, just 20 minutes outside Los Angeles. If it's summer, pair it with a half-day on the water slides. 25 is still young enough to have fun like a kid, and a group of friends would love an excuse to go have fun on roller coasters.
You could do something uber-hip and do a crawl of downtown LA bars. Downtown has undergone a renaissance in recent years, with some of the city's hottest bars and restaurants. Start at Golden Gopher, then head over to the Broadway Bar, then Seven Grand, and end on the roof of the Standard Hotel. If you want something closer to Hollywood, invite a group to The Shortstop in Echo Park, and play pool, drink the best martini in town, and dance to great d.j.s.
If you want something near the beach, do a crawl on Main Street in Santa Monica. Start at Chaya Venice, then go to the beautiful outdoor bar at 3110, then wander over to O'Brien's and end with dancing at Circle Bar.
Koreatown is home to amazing inexpensive and fun restaurants, with many korean barbecues. Everyone loves the food, and it is a fantastic idea for a 25th birthday, both casual and chic (and cheap).
If you decide to do something at home, create a theme. Maybe do a mexican dinner and bring in from Poquito Mas or Tacos Por Favor (super inexpensive and always good). Ask friends to bring their favorite tequila and do a tequila tasting (not shots, but sipping). As always, have a designated driver or use cabs.