Honestly, it can be anything as long as you are with the right person. Walking around the city, taking pictures of weird alley names, laying on grass in a park, crash on a couch and marathon lotr, cook or bake together. Even going to library and studying together in silence.
It doesn't have to be fancy or happen in a specific place like a café or cinema. Just hanging out, knowing the other person doesn't want the day to end as much as you do.