Hey great idea. Love the south of France, especially in July and August when everyone is on holiday.
It need not be expensive as many of the posters have mentioned. I've holidayed there a few times during the summer and due to the number of rooms something was available, just book in advance.
So my first tip would be to not split the trip between Nice and Cannes. Pretty similar in my opinion and you have the hassle of packing up and unpacking and losing a day to change city and hotel to just move a bit along the beach. If it was me again choosing between the two I would say Nice as it has better transport connections, is closer to the airport (i.e. cheaper taxi to your hotel) and from Nice it is easy to make day trips into the hills, to Monaco, and / or over to Italy.
I would have day trips to St Paul de Vence http://www.saint-pauldevence.com/en You could also take the train into the hills http://en.nicetourisme.com/nice/1342-train-des-merveilles
I would also look at a day trip to Italian coast - I recall San Remo and I stayed in small town Finale Liguria.
The shopping is very good in Nice. Eating out is a bit of a hassle as so touristic . On my last trip I had a great meal near Place St Francois.
Beaches near Nice I love Eze it's a long walk down (take your lunch and drink as nothing down there) and back but away from a lot of crowds. It is just outside the town in a bend in the road (bus stop and parking opposite, don't take the train). For whatever reason I prefer the beaches east of Nice.