With the exception of 'ski specials' Eurostar services run
solely to Calais (although very few trains stop there), Lille, Brussels and Paris (with a few Paris services running directly to Disneyland).
The ski services run to Bourg St Maurice, Aime La Plagne, Moutiers and Avignon.
An extra fiver each way onto the fare for Brussels allows you to travel to/from any other station in Belgium (using only the Belgian national train services, not ICE or Thalys trains). Since Belgium is a small country, all stations are under 2 hours away from Brussels, The coastal trains run from frequently Platform 15 and serve the beautiful city of Ghent, then the equally stunning Bruges before heading on to Ostend or other coastal resorts. (It usually takes under 1½ hours right to to the coast, with Ghent and Bruges obviously requiring shorter journeys).
Other popular destinations within Belgium are Antwerp and Liege.
The most popular destinations within a short travelling time from Paris are probably Rouen and Rheims.
Route map:
http://www.eurostar.com/uk-en/destinations/route-map
Timetable:
https://www.eurostar.com/sites/default/files/pdf/timetable/UK_timetable.pdf
The Eurostar site map is a good starting point for finding out about services to any particular country but (as explained above) you can't avoid changing trains at a Eurostar terminus:
http://www.eurostar.com/uk-en/sitemap