When I went to Provence and the Cote d'Azur in 2005, I bought the Rick Steves regional guidebook and the Lonely Planet regional guidebook. While I was happy with the RS book, I found the LP book to be more thorough and to include more things for younger adults. A new edition of the LP book has just come out. See here.
I suggest that you go to a good bookstore and take a look at both guidebooks.
I agree with Steve that Nice, Antibes, and Villefranche-sur-Mer would all be good choices. Since you mentioned the beach, you should know that Nice has a shingle beach (large pebbles). Antibes has a sandy beach.
If you were to tell us some of your interests, it would be easier to give you a list of suggested things to see and do. For example, if you are art lovers, you'll be happy to know that, outside of Paris, Nice and the surrounding area have the best art museums in France.