Make a treat together, then take it to a crabby neighbor. Leave a note, wishing them a happy day. My wife and I did this with our upstairs neighbor a few years ago. He was a stay up all night, smoke pot, play loud music kind of neighbor. After we took him some brownies, we never heard the loud music again.
Have a silly string war. Grab a few other couples, buy some silly string ($1 - $3 at a toy store) then go to a park and then set up two teams. Run at each other from opposite ends of the park. When you get close, start spraying. The person with the least amount of silly string on them doesn't have to help clean up.
Offer to babysit someone else's kid(s). Do it for an evening or an afternoon. If you don't have kids yet, this will be a good experience and tell you what it's like. If you do, then you already understand how rare it is to get a night away. You'll make friends for life (or at least someone who owes you), and you get to play with a child.