It will be really pleasant by September, also its not worth going in August as the whole city shuts down and everyone goes on holiday (seriously, Paris is dead in August its rather bizarre).
Best way to see the sights is grab yourself a bateux boat hop on hop off ticket, the boats run up and down the river dropping you off at the tower, louvre etc makes for a nicer way to go from one to the next. Oh and I would advise you go visit Ile Saint Louis, its the smaller of the two islands in the middle of Paris, I am biased as I lived there but it is a hidden gem and you will find Bertillion there which does the best ice cream in Paris