Just as an advice :

- 27€ seat reservation Brussels - Paris. Can sell out weeks in advance (quota)

- 37€ seat reservation Paris - Barcelona. Regular tickets are extremely expensive and some dates are already sold out for the summer. Book it ASAP

Alternatives :

- TGV Lille - Paris limited 10€, then 20€ (lots of availability, even last-minute)

- TGV Paris - south of France (Montpellier, Narbonne, Perpignan) 10-20€ 

- TGV Lille - south of France 10-20€ as above + reservation-free regional trains via Portbou

On one inbound/trip you could take a TGV from/to Lille + regular tickets across the border

On the inbound journey without a valid pass for Belgium:

If you use Thalys, you will need a normal ticket Paris-Brussels since for Thalys you need a valid pass for the complete journey.

If you use a TGV south of France to Brussels, then there's a pass fare for people with only a valid pass for France, but you need to book ASAP since pass holder seats on these TGVs are very limited.

Reservations are cheaper at the Belgian railways ticket offices because they don't have a booking fee on top of the reservations themselves.

If you use a TGV to Lille and then a normal train to Belgium, then you need a ticket from the border tariff points, so either Tourcoing(fr) or Mouscron(fr), to your destination in Belgium (buy at a ticket office). You can also use a Rail Pass of course from the first station in Belgium.

Nederlandstalig? Francophone ?

From which station do you leave from/want to return to in Belgium?

You can minimise reservation cost by taking reservation free regional trains from Belgium to/From Lille, take a TGV there to the south of France (see @thibcabe s post, although there are also some direct connections to the south, bypassing Paris from Lille. Very recommendable).

From the south of France, mainly Perpignan, you can continue with regional trains (TER en Rodalies, via Cerbère and Portbou) to Barcelona. If interested in this, mention your travel dates and this forum will give you good advice . Takes a bit more time though, but you'll avoid very pricey reservations and get better vue out of your pass

Starting in Lille for trains into France and beyond, an easy money saver, even with ordinary tickets, with not a lot if extra travel time. Especially if you live in West- or East Flanders.

If you want the quickest solution with the TGV Brussels-France, it's a very good idea to combine ordinary tickets with a pass. If you book ordinary tickets Brussels-Lille (or the other way around), these are usually around 19 EUR,  cheaper than the international reservation cost of an interrail reservation on that train. Reserving the same train from Lille to somewhere else in France, is only 10 (or 20 EUR). Saving your inbound/outbound pass days, for train companies where you get better value (basically any other than SNCF) 

"Inbound" and "Outbound" are terms used to describe the direction of movement of goods or services in international trade. "Inbound" refers to goods or services latest scrap metal prices coming into a country from another country, while "Outbound" refers to goods or services leaving a country and going to another country.

Belgium is a country located in Western Europe, and as such, its inbound and outbound trade can refer to the movement of goods or services between Belgium and other countries.