Croatia and Slovenia are on sale for under $500 round trip with SkyTeam Airlines for off season travel to their capital city airports. This is one of the best airfare deals I have seen to these places from SFO as someone who has had these two destinations in mind for the past couple of years. Ljubljana, SLovenia LJU and Zagreb, Croatia ZAG airports have discount fares from November to May. Split SPU has limited dates under $500, however, Zagreb has many low fare dates under $500 and intra-Croatia flights are relatively cheap from Zagreb at around 100 EUR round trip to several coastal cities like Pula, Split, Zadar and Dubrovnik.
There are many cities across Europe on sale with SkyTeam with the best far less common deals being to cities in Poland, Prague, Vienna, Budapest and airports around the Balkan nations. These are cities not common for discount fares from the west coast.
Air France San Francisco SFO – Paris CDG – Ljubljana LJU – Paris CDG – Los Angeles LAX – SFO
Alitalia San Francisco SFO – Atlanta ATL – Rome FCO – Split SPU – Rome FCO – New York JFK – SFO