We plan to take a train from London to Barcelona. I'd like to spend the day in Paris, than take the night train to Barcelona. I'm not sure what to do with our luggage for the day. Is it safe in the lockers? Also, how far from the train stations we have to transfer too, and how far from the train station to Paris sightseeing sites? Thank you.
You will arrive at Gare du Nord. I assume you'll be taking EN 477 that departs from Gare d'Austerlitz at 20:32 and arrives at Barcelona Franca at 08:24. It's a 14-minute ride on Metro line 5 from Gare du Nord to Gare d'Austerlitz. The station has both a staffed left luggage office and lockers. The lockers certainly are safe. It will be a short Metro and/or RER ride from Gare d'Austerlitz to any of the major tourist attractions in central Paris. For a zoomable map of the Metro and RER system, go here.
To get around on the Metro, RER, and bus, buy either a 10-ticket carnet (€11.40) and share the tickets or a zone 1-2 Mobilis pass (€5.80) for each person.