It may sound odd to PLAN something unexpected, but I take great joy in doing this with my kids. We love to plan simple surprises for them. They are so happy when we tell them to hop into the car and decide not to give away where we're going. It may be heading to a local pool, going to a movie, heading out on a geocache adventure or to grab some ice cream. My husband and I take care of the details and they get to go along for the ride. They are always excited and love guessing what our destination might be.
If you've never tried this, or haven't done so recently, treat your kids to the unexpected! They'll thank you for it!