Affordable spot to host a big birthday party?

We will be traveling to town for my partners birthday. Little does anyone know….ill be proposing at the same time! There will be 10-15 of us. It’ll be a total surprise so looking for a private feel where everyone can jump out and say “surprise!” But also quiet enough that I can give my little speech at the end and officially propose. I can’t go crazy, but hoping to find a restaurant, rooftop bar, sailboat, etc that would be a great surprise and add to the celebration. Ideas?