Travel to Syria is very dangerous due to a deadly civil war. See the main Syria page for details.
The Dead Cities are a dispersed collection of world heritage-listed in Northwestern Syria. These Roman and Early Christian towns towns once formed part of Antioch. They have long since been abandoned but make an interesting stop for tourists.
Northwestern Syria has numerous ruins spanning recorded history:
Both Serjilla and Al Bara are reachable from Aleppo (or Hama) by a combination of taking regular public microbuses and hitchhiking the last stretch. Take a microbus to the city of Ma'aret an Nu'man, and switch to another microbus headed for Kafranbel from there.
Once there, its probably best to ask for walking directions, and start walking the road to Serjillah. Try waving down any vehicle that goes in the same direction, chances are very good they will offer you a ride free of charge.
The same approach works for the way back, and anyone taking you for a ride is likely to drop you off somewhere on the microbus route. Make sure to head back in time though, these microbusses cease operation as early as 16:00.
The Simeon Monastery can be reached in similar fashion from Aleppo. Take a microbus for Daret'Azze and walk/hitch a ride from there.
Aleppo has several bus stations closely laid out in a cluster, and it can be a daunting task to find the correct point of departure for your destination. The best source of information are people that work at the stations (food and drink vendors, drivers). Some will try to sell you an expensive chartered vehicle ride, but many are helpful and will point you in the right direction.