Pay the Forfeit
Idea suggested by Selene from Thrace.
This surprise assumes you have a house available to be alone. You'll also need some time to prepare the surprise, so if your partner doesn't have to go out for any specific reason, don't hesitate to send them shopping ;) The ideal time to carry out the surprise is just when it's time for lunch, tea, or dinner.
The first thing you'll need to do is leave a highly visible note at the entrance of the house saying: "If you want dinner, I'm waiting for you in the room... don't forget to pay the forfeit." Prepare a path to the bedroom with several "appetizers" (chocolates, make them irresistible). The last one should be a glass of champagne and will be right before the bedroom door. For each "appetizer" your partner eats, they must pay the forfeit, meaning they must remove an item of clothing. Calculate the number of appetizers so that they arrive in their underwear at the bedroom. There you'll be waiting with an ambiance worthy of grand occasions: candles lighting the room, a bottle of champagne and a couple of glasses, and a multitude of delicious desserts on different plates.
As for you, you're part of the surprise, so you need to be up to the task. Show off your sexiest side and wait on the bed (since the idea plays a lot with food, a great idea is to wait wearing ONLY an apron). When they ask for utensils to eat, be provocative: "you don't need them" is a great response ;)
How you feed them and the rest of the evening is up to you... let your imagination run wild :)