You should spend most of your restaurant money in Paris, as they truly have the best quality food. Then focus on seafood in Marseille/Barcelona. As for London, just keep it cheap as it all taste the same. Actually they do have some good indian food there so maybe try that out.

Paris and Barca are just packed with places to visit, the ones in Paris are pretty obvious, but for Barca I liked : park guell, sagrada familia, olympic port for the beach and just walking around, and la rambla for touristy stuff.

Marseille there's not much to do besides checking out a few small museums, but just walking around and eating good seafood should be fun enough.

Kinda jealous, sounds like fun and it brings back good memories! Enjoy.