Let's start with a few basic bits of information:
1. Any decent bookshop should be able to sell you the Thomas Cook Rail Map of Europe. It's a standard reference work for planning European rail travel. You probably don't really need it for one short trip but it might be useful for planning futures journeys. (It will also be available in larger public reference libraries)
2. All European rail timetables are available, in English, on the German Railways website:
http://reiseauskunft.bahn.de/bin/query.exe/en
3. Some international rail services need to be pre-booked. If you book through National Rail's official partner company, Rail Europe, you'll have to pay a booking fee. Once again, it's the German Railways which come to the rescue. Book your travel through their UK booking centre:
http://www.bahn.co.uk/-S:PtVOZ9:ecB6SdNNaIlmjN NNNUAM/db_uk/view/info/index.shtml
A quick glance at the Thomas Cook map shows that the distances from Brussels to either Italy or Russia are far too great to allow for you to travel there and back in such a short space of time. However, if I assume that you're travelling on the 0743 Eurostar, ex Waterloo, you could take the 1125 service to Lyon (arrives 1505) and then the 1533 train to Geneva (arrives 1719). That would give you some time to explore Geneva:
http://www.geneve-tourisme.ch/?rubrique=000000 0000&lang=_eng&PHPSESSID=cacaaf4764514d1d1a078 ec26ef2fb3c
If you want to visit a country other than Belgium, why not travel to Germany. There's a good train service to Cologne, with a typical journey time of around 2 hours