Jay-Z's 99 problems don't include party planning. The New York Post says the music mogul tossed a small shindig on Saturday in honor of wife Beyoncé's 29th birthday. Guests included Diddy, Alicia Keys, Kanye West and Serena Williams. (The chanteuse's sister, Solange, and her dad, Matthew Knowles, were apparently no-shows). "Jay brought a six-layer cake out for B; they were really affectionate," recounts a spy. "[DJ] Cassidy played Stevie Wonder's 'Happy Birthday.' The party went on until 6 a.m. ... Diddy was the last to leave. Jay filled the room with orchids." And what about that report claiming Jay-Z also plunked down $20 million to buy the birthday girl a private island in the Florida Keys? Beyoncé's rep didn't get back to us, but a private island seems more like a 30th-birthday kind of present, doncha think?