I would recommend one of the shorter-cooking items, those being ribs or chicken, chicken being the easier. Whether it's whole or halved, do yourself a favor and get an instant read themometer-- chicken is actually not that forgiving. The difference between moist and dry white meat can be as little as 5 degrees. If you do your chicken low (225-250*) it will take about 4 hours. When it hits 160* in the breast, take it off and rest it, loosely covered in foil for 15-20 minutes before carving. Alternately, you can do it at 325-350* with a dry water pan, which might work better on a new WSM, since they tend to run hotter until you get a soot layer built up on the inside. Again, go by your thermometer, and not your clock.
Ribs, specifically spares, can be cooked at 225-250* for anywhere from 5-1/2 to as much as 8 or 9 hours and still come out good. Splurge on baby backs, and you can be done in as little as 4-5 hours-- they're smaller and leaner.
Oh, and whatever thermometers you use, test them for accuracy in boiling water.