One idea for birthday party ideas for adults that are easy to come up with is a potluck competition. This can be fun idea to get together all of the adults in the house. This leads to an exciting event that they'll all enjoy. Everyone will come up with a dish to bring to the party. To avoid everyone bringing the same item, give each guest a course, like an appetizer, main course, pasta, meat, vegetarian main, dessert, etc. The guest of honor can judge the competition and pick a winner. Everyone can show off their cooking style, and you will save money on the food.