Here's a neat tidbit about that very familiar Advent hymn, "O Come, O Come, Emmanuel":  the words are based on the O antiphons, which have been sung since the very early days of the Church.  According to CatholicEducation.org, each of the antiphons is one of the titles of Christ:

O Sapientia (O Wisdom),
O Adonai (O Lord),
O Radix Jesse (O Root of Jesse),
O Clavis David (O Key of David),
O Oriens (O Rising Sun),
O Rex Gentium (O King of the Nations),
and O Emmanuel.

Here's the neat part:  look at the first letter of each title.  From first to last, it reads SARCORE.  Now read the letters backwards, and you get EROCRAS.  So what?  ERO CRAS is Latin for "I am coming," and you have to read it backwards because the source is at the end of the season.  It's a message spoken by someone waiting at the end of Advent -- and that someone is, of course, Christ Himself.  I am coming!

Love, love, love getting a promise, a reassurance -- and a warning! -- from the Savior before He is even born.  It reminds me of the deliciously baffling idea that Mary's Immaculate conception came about because she was already saved by the Savior before He was born, before she consented to his conception, before she was even born herself.  Theological mind-benders like this are a nice reminder, in case you needed it, that Christmas isn't a Precious Moments bedtime story that's only for kids and sentimental folk.  Christmas, and Advent, is just as much for adults as it is for kids.

Still, many people who are childless or single feel left out of what seems like the essential observances of the season.  But all the fuss and bustle of Christmas with kids can actually distract us from the main purpose of Advent, which is to prepare for the coming of the Lord.  Here are some ways that adults can participate meaningfully in the preparation for Christmas:

Use candlelight. Even if you don't have an advent wreath at your house, light a candle at dinner or in the evening, and let it restore you to some sense of peace and quiet. 

Switch gears on music.  If it's on the radio at this time of year, it either stinks on ice, or it's been played so many times by Dec. 1 that you'd rather listen to hogs being slaughtered than hear it one more time.  Queue up some chant, instead.  It's prayer, not entertainment, and listening to it pleases your ears and reorients your soul.  Grooveshark's "advent music" channel is pretty good

Get thee to a confessional.  If you do nothing else in Advent, do this.  There is no better way to prepare for the coming of the Lord than to clean house, spiritually.  Here is a basic primer on how to make a good confession ; and here is thorough examination of conscience that helps us do a "deep cleaning" when we take stock of how we spend our days.

Finally get around to that spiritual reading you've been meaning to do.  Try turning off your computer or TV half an hour earlier than usual, and spending that time on spiritual reading.  I finally bought Francis de Sales' Introduction to the Devout Life, and I'm starting it tonight!  Other reading recommendations I've gotten: Interior Freedom by Jacques Philippe; Wood of the Cradle, Wood of the Cross by Caryll Houselander; Self-esteem without Selfishness by Michel Esparza; Redeemer in the Womb by Fr. John Saward; To Know Christ Jesus by Frank Sheed; and What It Means To Be A Christian, which is three Advent homilies by Joseph Ratzinger.

Of course, you can also spend the rest of Advent slowly reading and digesting the Pope's apostolic exhortation, Evangelii Gaudiam.

Form a new prayer habit.  You could use the rest of Advent as a time to learn to follow all or part of the Breviary.  Daria Sockey of Coffee and Canticles recommends iBreviary,  DivineOffice.org, or Universalis.

Be alert to nesting impulses.  When a woman is almost ready to give birth, she is often seized with an irrepressible compulsion to clean and prepare --  in obvious ways, such as stocking up on diapers and baby clothes, and in ways that have no apparent connection to the impending birth, such as alphabetizing the pantry or polishing lightbulbs.  Pregnant women are on a constant alert to prepare in any way that they can.  We can all do the same:  make a conscious effort to be open and ready to do what God wants us to do to prepare for the birth of His Son.  Be alert!  Keep your eyes, ears, and heart open, and you will find ways to prepare.