fly to Paris/ train to Barcelona/ fly home from Barcelona in Sept.

11 nights ??
Easy, 6 in Paris, 5 in Barcelona.

What a nicely paced trip! You'll get a chance to see a fair amount of two cities - great places, too....two of my favorite cities!!

Thank you very much.
Any tips or "must see" sights greatly appreciated.

We are also traveling to Paris, Amsterdam and Barcelona. My daughter wanted Amsterdam so we have a travel day plus 3 full days in each place. We are taking the train from Paris to Amsterdam and flying to Barcelona from there. Vueling has great airfares if you want to check into it.
I would like to have more time in each place but this is what we have, plus some days before and after getting my daughter settled in Spain for her study abroad.
If you wants to share ideas you have for Paris or Barcelona, message me.

As David mentioned, is this 11 nights or is day 11 the day you fly home?

For Paris, this can be a value. http://en.parismuseumpass.com

Try to group the sights for each day as close together as possible. Then you're not hopping all over the city unnecessarily. Note evening openings for some of the museums.

I would fly
https://m.vueling.com
Paris Orly- Barcelona. Many flights per day and much less expensive than an 81/2 hour train ride.

This is actually a nice route by train. I just did it. There's one that leaves Paris at 10:07 and gets to Barcelona at 4:42, so 6.5 hours. City center to city center, so no airports, no showing up early and waiting around. It was a nice train ride.

I'll be doing the 6.5 hour TGV Paris to Barcelona in October and am looking forward to the relaxing ride and the scenery. I've taken the TGV before for shorter distances and found it to be comfortable and enjoyable. Much less stress and hassle than getting out to Orly or CDG to go through flight security etc etc