Well it's good you don;t want to stay in one place since Paris is nowhere near a beach. You might consider fluing into Paris, staying 5 days there , then moving tothe Riviers (mobbed) or cost near Deauville (also mobbed and cooler) for 3 days before returning.

that is assuming you have 10 days. If you only have 8 days (and does this assume the day you arrive and depart? if so, you really only have 6 days) then I would just stay in Patis and make 1 or at most 2 day trips to see the countryside. (And I believe they make a faux beach on part of the Seince during the summer - have no idea what it is like.)

The aspects of France that made you choose it would be your must-sees. Get a guide book and do some research and then come back with questions that are a bit more specific.

For beach time consider areas consider some of the areas on the west coast. Even areas as far north as Brittany have some lovely beaches. If you are flying in and out of Paris I would head to the beach directly from the airport and would choose a location where I could train directly from the airport as you have limited time. Then head back and spend the rest of the time in Paris then fly home.

Keep in mind the beach places will be packed in August!

Enjoy!

http://www.paris.fr/parisplages

Paris Plages is on until august 19th this year. That's the faux beach, and let me assure you: it's absolutely brilliant. Paris will come out of its summer sleep in august , la rentrée, also fun, But be aware that if you come in early august some places, shops, bars, may not have opened up yet. So if your dates are before the 19th, I wouldn't bother with going west to a beach, I'd just use Paris Plages as the beach destination. I've done it with my son, and there's really something for everyone. Also a perennial hit is the science museum:

http://www.cite-sciences.fr/fr/cite-des-sciences/

Museum doesn't really describe it: it's completely geared to young persons and usually it is the one "end" of Paris Plages, with sailing courses going on in the bassin de la villette.

Swimming pools in Paris can be equally nice: for instance Pontoise, which is in a lot of movies. Just make sure you wear the regulation cap and trunks and you'll be ok.

The 11th is nice, Canal St Martin, Voie Plantee, too.

The museum at Quai Branly, the new ethnographic place, can be interesting too, but is highly ideological, the one thing my son found absolutely irresistible are the walk in music boxes on the first floor and the building has a very intriguing design.

And for a completely off the beaten track park experience I recommend Parc Andre Citroen, on the site of the old Citroen auto plant. You can do a balloon ride (straight up and down) that will give you a better vantage point to see Paris than the Eiffel tower does, and I can guarantee you there will be almost no tourists there.