* If you don't already have a midweek Mass, that is the place to start. See if you can parlay these Advent services into a regular midweek low Mass. This is of great spiritual benefit to the congregation if for no other reason than to allow vacationers, shift-workers, etc., to catch a Divine Service if they have to miss Sunday. For propers, see DDSB.
* If you already have a midweek Mass or for some other reason want to keep this a prayer service, use it as an opportunity for the congregation to learn Vespers, Evening Prayer, or Compline.
As for liturgical themes, readings, psalmody, etc., here are three ideas to consider that avoid kitsch and idiosyncrasy, which often goes by the name "creativity" these days.
1. Saints. Andrew (Nov 30); Nicholas (Dec 6); Lucia (Dec 13); Thomas (Dec 21). One each for the four weeks of Advent with obvious tie-ins to the season and historic readings (which can be found in DDSB). That's hard to beat.
2. Treasury of Daily Prayer. This is our Lutheran Breviary and is wonderfully done. Take a look at the assigned readings for the weeks in Advent. Let the Writings guide and inspire your preaching. Also take a look at a Roman Breviary for more meditations from the Fathers as well as a little perspective on the history of the TDP's readings. For the actual service, I think picking one day's readings from the week for the service is the best idea, to go along with encouraging the whole congregation to take up this discipline in the home.
3. Previous Sunday's Epistle lesson. Traditionally, the Gospel serves as the text for the Sunday sermon with the Epistle playing only a supporting role. These midweek services are a fine time to preach on the Epistles, or LSB's OT lessons which are nicely focused on prophecies of Christ.