Barcelona in June 2015

With a slight pinch of salt taken, in Europe you can fly from pretty much everywhere to everywhere else at will.

Random example; leave Frankfurt immediately on arrival (because there's not that much to it), and walk round to check in for a flight down to Italy. With 12 days, you could go to Venice, do that for 3 days, head down to Florence for 2 days, down to Rome for 3, fly over to Barcelona for the rest of the trip. For running around Italy, use internal trains (see http://www.seat61.com). For flights around Europe, cross reference Expedia with http://www.skyscanner.net for the budget airlines. Bear in mind that Venice Treviso, Barcelona Reus and Barcelona Girona airports are quite remote from your actual intended destination.

If the only criteria is a city between Frankfurt and Barcelona, then there could be hundreds, but since your question is very vague and you haven't given any clues about you interests, it could be hit and miss.

And why not Italy if that is where you want to go? Have you done any research into the feasablity of this apart from looking at a map? like transport time and options between Italy and Barcelona?
Frankly i think you need to do a bit of homework yourself and not expect strangers on a travel forum to do it for you.

I would suggest to stay in Spain and visit some more places and not only Barcelona. Madrid, the capital town of Spain, is another great town to discover (I personally prefer Madrid to Barcelona, but that's just my opinion...), such as Valencia, Bilbao, Siviglia, Granada and all the Andalusia region is very beautiful.
Other interesting places to visit in Spain are the rock village (Setenil de las Bodegas), the Natural Park of Montsant, the National Park of Garajonay, Las Medulas and Cataluna Region in general (I love it!)
Another option is to go to South of France and Cote d'Azur from Barcelona, maybe also until Liguria Region in Italy, passing by Nice, Marseille and Cinque Terre, maybe then coming back by ferry to Barcelona so you don't need to drive all the way back.
Let us know what will you decide! :-)

We would like to go to places where there are many tourist attractions. We want to maximize our time in Europe,by means we don't want to spend too much time on the road and also allocate a lot of budget on the transportation so that is why we plan to go to cities along the way to Barcelona. 3-4 cities will be great.

Any suggestions about this itinerary?
Frankfurt-Munich-Viena-Barcelona or Frankfurt-Venice-Rome-Barcelona or Frankfurt-Lyon-Montpellier-Barcelona

Thanks in advance

I read some of the entries that there was not many things to be seen in Nice and Marseile but I will find more information about it.. Thank you so much for your suggestions..

We would like to go to places where there are many tourist attractions.

Could you please define what are 'tourist attractions'?

You will have 12 days minus 2 days for onward/return flights minus at least 3 days for Barcelona. In the remaining days you might visit just 2 other places (3 days per place plus 1 day for transports).
You may take a train from Frankfurt airport to Stuttgart and fly from there to Rome. Fares start around 40 EUR. You could then visit Rome and another town in the area (Naples, Perugia, Florence or so) and finally board a comfortable Barcelona bound cruise ferry (at Civitavecchia or Livorno).
http://www.grimaldi-ferries.it/en
For cheap flights to Barcelona, check http://www.vueling.com/en

It really depends on what you like... I love both Nice and Marseille, and the whole Cote d'Azur is a really interesting region, even when the weather is not so hot to go swim into that beautiful sea. it is also a very touristic area, as you are looking for. also italian Cinque Terre is very touristic and you will find many colorful villages... Personally I won't go all around Europe in only 12 days, I would find it stressful and very time-consuming. I would rather focus on just one country (Spain) or just one area (Barcelona - Cote d'Azur - Liguria) to enjoy it properly, have a great time, deepen my knowledge on that area and also have the time to meet some locals who would show me around...

Frankfurt-Lyon-Montpellier-Barcelona

This is the most logical route, but Lyon and Montpellier are not necessarily the most interesting/beautiful towns of this route.

I'd stop first in Alsace for a couple of days: Strasbourg, Colmar and the cute small towns like Riquewihr, Kaysersberg, etc.

Than a couple of days in Provence: Orange(just a couple of hours to see the Roman theatre), Avignon, Pont du Gard, Nimes, Arles, etc.

Than a short detour to Carcassone.