Train travel

Start here:

https://www.seat61.com/international-trains/trains-from-Paris.htm#Paris-Barcelona

if you don't have tickets, this is a route to fly. The train trip will be long and boring. If you do that, be sure to stock well with picnic supplies.

The train trip is only marginally faster than flying, and not boring at all.

I've done this both ways -- next time I'll fly.

when the overall time is similar, the train gives you opportunities for walking up and down the train, visiting toilet facilities which are larger than a thimble and don't come with exotic sound effects. Tables, plenty of legroom, and something more scenic than looking down at clouds.