Delta has round-trip flights from Houston (IAH/HOU) to Milan, Florence, Venice, Lisbon, Geneva, Cork, Rome, Budapest, Copenhagen, Krakow, Athens, Munich, Luxembourg, Bucharest, Frankfurt, or Berlin for $488-$593 with a connection. Flights October to December.
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.