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The question remains very vague, but I will make a few suggestions.
If you want to avoid Paris coming from Belgium then consider a stopover in Lille, or Lyon (both have multiple direct trains daily).
If you want to play around with itineraries with long stopovers, then I (ironically) recommend http://bahn.de, which has French timetables included. You can add up to two stops of up to 48 hours each. It won't give you prices though. Plenty of other sites will let you check journeys Brussels-Lille and then separately Lille-Destination. This may well be easier for you.
It is unlikely to work out much more expensive to buy two or three point-to-point tickets for each leg of your desired journey, I don't think SNCF will let you deliberately extend stopovers on a single ticket.
Try wikitravel for suggestions of nice places for a day, but be wary of towns that are far from the station. (A clue is if 'TGV' is in the name of the station: Avignon TGV, Aix-en-Provence TGV are all a bus ride out of town, so a pain for a quick visit.)