Oooh, I've been meaning to answer this one!
I've had a lot of perfect birthdays since mine is in the height of summer. I went to Mackinac Island in Michigan for one, to an aquarium, zoo, and Mammoth Caves in Kentucky for one, I spent two of them at Yellowstone National Park when I worked there, I had a party up north camping a few years ago, and I've had countless parties with my friends at home. Last year was incredible, I had a Leo-themed birthday (lol), and all of my friends came, several brought their dogs, and they (the humans) brought me the most thoughtful presents. We had a homemade cake, played in the pool, and watched Jurassic Park on a projector.
Either have a party with friends or go with one or two friends on an adventure to somewhere I love or somewhere new.
Ooh, what about renting an island for a week and bringing all of my friends to hang out, drink pina coladas, go snorkeling, and relax in the sunshine. I know that's super basic, but I think that would be so fun, and I love the ocean. I also like the idea of a haunted mansion that @SwampCreature said, OR renting a castle in Ireland and having a LARP with friends. Maybe that would be my actual one, I have always wanted to visit Ireland, and I know I would love it.