A cooking class together can be both fun and romantic. You can learn to make a new cuisine, laugh at each other's cooking mistakes and at the end, enjoy the meal you've created. It gives you a chance to bond over a shared activity and learn something new at the same time.
A picnic in the park. You can prepare some delicious food and drinks, enjoy the natural scenery while chatting.
Going to a local theater to watch a play or a musical. It offers a different kind of entertainment experience for the two of you.
Another interesting OC could be a spirit who has the ability to communicate with animals. This OC could bring a whole new dynamic to the story as she tries to understand the human world through the eyes of the animals she befriends. She might also use her animal friends to help Shido in his missions, like having birds spy on the AST or something.
One idea could be that the main character has memories of their past lives and uses that knowledge to change the fates of the Spirits in a more positive way. For example, they might be able to prevent some of the more tragic events that usually occur in the original story.
One idea could be exploring an alternate universe where the Spirits and the main character have different initial meetings. For example, instead of meeting in a battle - like situation, they meet in a peaceful school event. Another plot could involve a Spirit who has lost her powers and the main character has to help her regain them without using the usual methods. It could lead to a whole new adventure and deeper understanding between them.
One idea could be exploring an alternate universe where the spirits have different powers and relationships. For example, making Tohka have the power to control time and seeing how that changes her interactions with Shido. Another could be a story focused on the backstories of the spirits before they became spirits, like what Kurumi's life was like as a normal human. Maybe a fanfic where Shido has to go through a series of trials to prove his love for the spirits in a more extreme way than in the original story.
A picnic in the park. You can prepare some sandwiches, fruits and a bottle of wine, and enjoy the fresh air and beautiful scenery together.
Another idea is a cooking class date. They sign up for a cooking class together, learn how to make a new dish like a foreign cuisine. They have fun cooking side by side, and at the end, they get to enjoy the meal they prepared together.
One idea could be that they go on a magical picnic in a hidden meadow in Hogwarts grounds. They use magic to create floating plates and self - serving dishes of their favorite foods. Another could be that they attend a wizarding concert together and discover a newfound love for a particular type of magical music.
Well, a really cute plot could be that the girl and her date are both into art. They go to an art museum for their date. As they stroll through the galleries, they discuss their favorite paintings and artists. Then they end the date by trying to paint each other in a nearby park. It's a sweet and creative date idea for a fanfic.