Delta has round-trip flights from Houston (IAH) to Istanbul, Casablanca, Zagreb, Warsaw, Vienna, Stockholm, Prague, Zurich, Porto, Naples, Gothenburg, or Gdansk for $504-$595 with a connection. Flights October to March. Extended weekends available.
Other destinations in Europe are also on sale. Smaller airports (not listed here) may have long connection times or multiple stops.
A Carry-on bag and also a personal item is allowed for free on Delta. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.