How to extend Toulouse trip with a toddler?

It’s been several years since I’ve been in that area (Nice and/or Lyon have been my primary places to visit in France the last few trips), but the Dordogne would make an outstanding second area. For a bigger town, I’d consider Sarlat-la-Canéda, and for a smaller, charming home base, Beynac-et-Cazenac would be my suggestion.

We love Collioure and highly recommend it. A charming town, filled art history, a beach and good restaurants.
We also love the Basque regions in Spain and France. Look at Hondarribia as a base. It is near the Spanish border with France and you can tour both the Basque regions easily from there.

I assume that your flight will arrive in Toulouse Blagnac, and your return flight will also be from Toulouse.

In order not to spend all your time in a car, you will have to make choices.

If you are heading towards Carcassonne, Collioure etc, you will not have enough time to go to the Basque country which is more than 5 hours by road from Collioure and its region.

You will also be quite a distance from the Dordogne, so save this area for another time.

Moreover in Saint Lizier you will be in the department of Ariège where there are plenty of things to see and do.

Albi is in Occitanie (and not in Languedoc Rousillon) and is the capital of the Tarn department. it is about 2 hours from St -Lizier via Toulouse. It can be a day trip or you can also spend a night there.

Carcassonne is geographically closer but it will take as long, if not longer, than going to Albi. On the other hand, the road will be more pleasant.
You can stay one night in Carcassonne and visit the medieval city, no need to stay longer.

From Saint-Lizier, on the road to Carcassonne, you can visit the Cathar castles of Montségur and Roquefixade, to name just these two. The landscapes and the roads are superb.

If you absolutely insist on driving for hours and being stuck in traffic jams in the sun with a 2.5 year old child, from Carcassonne you can easily reach the entire south coast, from Narbonne to the Spanish border.
Be aware that in July there will be a lot of people on the coasts and on the beaches. (Argeles sur mer, St Cyprien are invaded by tourists in July and August)

Collioure will also be full of tourists, so to find a room plan to book months in advance.
Note that if you like anchovies, Collioure is where you can find the best ones.

You can also continue to travel along the coast towards Spain via "Port Vendres" then "Banyuls sur mer"
In Banyuls no anchovies but a sweet wine specialty called "Banyuls", which is drunk as an aperitif.

The French coast ends at the town of Cerbere which had its heyday when the trains loaded with fruit from Spain circulated there.

These comments are so helpful! Yes, current plan is flying in and out of Toulouse. Based on all your ideas, I put together one potential itinerary of 4 nights in Saint-Lizier for the wedding, 3 nights in Villefranche-de-Conflent, 3 nights in Collioure, and 2 nights in Carcassonne. Does that sound like it would be enjoyable/well paced with a toddler? Or too crowded with tourists? I’m also interested in putting together an alternate itinerary where still start in Saint-Lizier, but instead of VdC/Collioure/Carcassonne we would explore Basque Country. I’d love ideas for a similarly paced route through there. Thank you all!

We did similar in reverse, mid/late July travelling by train. This was pre covid so maybe things are a little different now.

Carcassonne. The Citadel is very crowded within the walls during the day. We did the self guided tour of Chateau Comtal (you have to pay) around midday, this gets you access up on the walls, not too crowded. We then went and had a picnic lunch outside beside the walls, no crowd issues. We wandered around inside the walls in the early morning with hardly a soul around and late afternoon/evening, quite a few around but not crowded.
In town itself no issues with crowds except the day the Tour de France stage finish hit town. Even then we were able to walk up for a table in Place Carnot in the evening and sat at the bar at our favourite pub later.

Collioure. It was 35 C each day we were there. Crowded in the alleyways and at the eateries. The main beach had plenty of people but not super crowded. I looked at my photos and could count around 100 people and 8 sun umbrellas. I did plenty of swimming back and forth and crowds were no issue. There is another beach on the other side of the church and it had less crowds. There are some other small beaches that can be accessed too.
We only booked our hotel at the Tourist Info around midday the day we arrived (Annexe of the Templiers Hotel). With a little one you probably want to be more organised 😆. It was a comfy 2 star in good locale, brekkie was served in the famous Templiers Bar. Not sure if it is toddler friendly. Most of the sights (Chateau Royale, the ancient windmill and Fort St Elme etc) were all uncrowded when we visited. We caught market day in both Carc. and Coll. Bonus!

Villefranche de Conflent. Plenty of daytrppers but certainly not unpleasantly crowded. The streets were empty by late afternoon. We only saw 2 other couples when walking the ramparts.
No crowds in the 1000 steps/tunnels. Some small tour groups at Fort Liberia but no crowd issues.
The Grande Canalette is pretty big plenty of tourists but plenty of room, we had a guided tour with about 10 others through another of the caves.
We stayed one night at the old post office BNB loved it. A bit above our usual bargain basement hotel price, other nights were at a grand old unloved hotel outside the walls, more our price range.
The roofless carriages of the "Canary" were fairly crowded for some of the journey. It was a fun journey with a friendly crowd.
There are a number of other places around Villefranche de Conflent that we couldn't access without a car.

Villefranche-de-Conflent is a good base to stay and visit the region.

Anyway be aware that this region, Saint-Lizier, Villefranche, etc is a mountainous region with lots of slopes, steps and stairs in towns, villages and places to see. The toddler could sometimes be more of a problem than an advantage.

The medieval city of Carcassonne also has a lot of steps and stairs, especially along the fortification ramparts (in the Middle Ages they had not yet invented the elevator), but the rest of the medieval city is ok with a toddler. Your child will be just a little shaken as these are cobbled streets and alleys.

The seaside (Collioures, etc.) are less problematic

I was surprised to see several folks mention Villefranche-de-Conflent, which is a bit off the beaten path. Just in case it's useful to you or later readers: V-d-C is the starting point for the Yellow Train through the Pyrenees to Latour-de-Carol. It's a scenic, though rather slow, ride. I highly recommend it. A round trip is possible over the course of a day. V-d-C was overrun at the time of my visit (midday, because I had started near the other end of the route), but I still enjoyed seeing the town. If you drive there and have some control over your schedule, it probably is smart to avoid midday unless your reason for being there at that time is to have lunch.