From Barcelona · 41.39° N · 2.17° E
Eight getaways for 11 September from Barcelona: the grape harvest, rice nearly ready to cut, the warmest sea of the year and destinations outside Catalonia that work that day.
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In France 11 September is an ordinary day and families are already back at school, so the Cité is emptier than in August. The Canal du Midi boat leaves in September at 10:30 and 14:30.

The Diada is a working day in Aragon, and in September the Matarraña comes out of the summer heat. The Parrizal ticket is bought by time slot; kayaks on the Pena reservoir continue until mid-September, so on Sunday the water is still worth it.

The Saturday historic train returns from the end of August, with period carriages. Go up to Lleida by high-speed train from Sants; Collegats rafting has already closed by September, but the Roca Foradada via ferrata is better done without the heat.

In September it gets dark earlier than in summer and the astronomical park session starts at a more convenient hour. The Prades forest, at almost a thousand metres, is cool for the Baltasana; book the session and check the moon.

The Priorat harvest fills September and October: the vineyard with Devinssi shows the grape still on the vine or freshly cut. At harvest time everything is booked weeks ahead, and Amics is closed on Wednesdays.

In September the Pals rice fields are golden, close to a harvest that comes between the end of September and mid-October. The rice train has limited places, and the beach is no longer the August one.

September has the warmest sea of the year and Garraf beach keeps its lifeguard until October. The R2 Sud stops on the sand, and away from the August Sundays the village has room again.

On 11 September the Garonne can still be rafted, with less water than in June; confirm the departure when you book, because outside high season it requires a minimum of three people. The Diada is also a holiday in Aran: book lunch.
Questions
Yes. In the Priorat and the Penedès the harvest takes up September and part of October. Book ahead: small wineries are working flat out and have few places.
Everything, because in Aragon and France 11 September is a working day. The Matarraña and Carcassonne operate like any other day, without the holiday crowds.
Yes, and the water is warmer than in June. Garraf, on the R2 Sud, has a lifeguard until October and far fewer people than in August.
Leave the evening before or early on the day itself. In the afternoon there is usually a demonstration and closures in the centre, and the return on the last day coincides with the end of the holidays.
Geocaliahttps://www.geocalia.com/en/spain/catalonia/barcelona/getaways/diada-long-weekend