8 nights in Spain

Pretty easy, IMHO. You really only have time for one other place (if that...) on a trip this short. I'd just get a cheap flight to Madrid and spend your 3 days there.

If you could change your flight in or out so it goes through Madrid instead of Barcelona, that would buy you another day or so (no need to double-back to BCN, a big waste of time on such a short trip). It's especially critical to avoid doubling back on such a short trip.

If your heart is really set on France, try for a cheap flight to Paris instead, but just remember you're really only going to have a 2 or 3 days maximum in each place - you can't count any day that you're arriving or departing (or moving between destinations). After you come to grips with that reality, how many FULL (non-travel) days do you really have to spend? If it's 6 days or less, I'd just stay close to Barcelona and enjoy that city and its surroundings.

I'll agree with David about a flight to Madrid. It's cheap to fly and very different than Barcelona. My son who lived in Spain says Barcelona is like New York (artist, creative) and Madrid is like Washington (historic, seat of government). I did this trip two years ago except I flew into Madrid and then on to Barcelona for a few days. After watching Mario Batali's "On the Road in Spain" I think if I did the trip today I'd probably consider renting a car and going into the wine country to see some of the countryside and experience rural Spain.

I appreciate your comments. I also watched On the road, Spain and became interested in the Countryside but thought it might be very hot to be inland mid-July. Madrid I hear is REALLY hot mid July so skipping it this trip. What do you think about taking a train to Provence then renting a car and working our way back to Barcelona?

I highly recommend going to Northern Spain. San Sebastian is a great place - any place in the Basque country

Alex,
I echo what others have posted about your proposed train trip to Paris -- too much time and hassle. Forget your worries about the heat, BCN is also wretchedly hot and humid in July (think Philadelphia), and this year is shaping up to be one heck of a summer. Instead, do as the natives do, and adapt your rhythm to that of the heat. Get up early for trips around town, take a nap or hang out on a terrace during the mid afternoon heat, and then spend much of the wonderful, silken-aired evenings exploring the plazas, bars and streets of Spain. A brief trip to Madrid is a possibility, but I would probably spend the time closer to home-base, instead.
From BCN you are a stone's throw from Gerona and Tarragona, as well as a day on the beach that would be difficult to achieve in Denver. Gerona has an unique (to my knowledge) Jewish museum documenting the massive pre-diaspora impact on Spain. There you will find guides as well as a nice walking tour. Gerona is an easy day trip from Barcelona (as is Tarragona.) Rather than renting a car, the train is quite adequate for this sort of excursion. While in Gerona, you might want to consider taking a trip down the coast to L'Escala to visit Empúries. As first the Greeks, then the Romans, and then early Christians came to this aptly named emporium, they settled adjacent parcels of land. All of this is remarkably well preserved and excavated, within earshot of the waves lapping a gorgeous beach. Besalú, also close by, is a "living museum" medieval town.

Tarragona is small and definitely worth just strolling around, especially if you have any interest in Roman history.
Unless you are desperate to cross France off of your list of countries visited, I wouldn't bother with a rushed, sweaty visit until having explored more immediate surroundings first.