If you usually spend your Sunday nights sitting around and dreading the fact that Monday morning is just around the corner, I highly recommend getting yourself to the nearest imax theater and catching the U2 3D concert movie.
Movies in 3D are crazy anyway, but this was like nothing I had ever seen.
I also recommend spending your Sunday afternoon with a good friend, wandering through a local bookstore and then sitting in a coffee shop for a while. Neither requires conversation, but both offer the opportunity for interaction if desired, which is a nice. It's also fun to wander around together and look at the drunk St. Patrick's Day revelers, then go get dinner before the movie.
And before all that relaxation time, I highly recommend doing lots of productive things like paying bills and grocery shopping and cleaning the kitchen and doing laundry. And if you can find the time to purchase a new hair product that makes your hair feel silky-smooth, then do that too.
In all, a great Sunday and one I'd repeat again anytime.