Fun ideas for a 40th birthday party include hosting a theme party, renting a banquet hall, renting an activity facility, holding an informal gathering, and giving the guest of honor the chance to do something she always wanted. The most suitable idea depends on the personality of the guest of honor.
One fun idea is to host a theme party. Popular themes include the Old West, a casino night or a Hawaiian luau. Alternately, the theme may be a time period relevant to the guest of honor's life, such as the '70s or '80s. The theme should be reflected in the party decorations, costumes, music and themed food.
An unexpected fun activity is to rent a banquet hall and use it to conduct a "roast" of the guest of honor. This includes putting together a funny and embarrassing collection of pictures and videos of the guest. Inviting close friends and relatives to provide amusing anecdotes about the guest of honor adds to the fun. A simpler version of this is to host an informal gathering in someone's backyard.
Another idea is to rent out an activity facility for the night. These include bowling alleys, water parks, movie theaters and swimming pools. One final idea involves taking the guest of honor to do an activity she always dreamed of, such as skydiving or scuba diving. This encourages the idea that life is only beginning at the age of 40.