Most Oneworld carriers have round-trip flights to various destinations in Europe at a discount today for $436-$630, mostly with a connection. Dates vary, use the linked calendar. Cheapest fares are mostly midweek. Most fares are not reversible.
Notes: Most destinations have flights from August to March available. The exact ranges for each destination will vary. Use the Google calendar view to get an idea of what dates work.
Flights to smaller airports may require a long connection or two stops. When you begin your search, modify your filter to remove fares that have more than one connection, some of those smaller airports ONLY have flights with bad routing.
