Whatever the season, a day in Nice is never a bad idea!
The old town is a true must see. We've already recommended the Vintage market on Cours Saleya on Monday mornings, but you also might want to check out the Marché aux Fleurs (Tuesday through to Sunday)
Get lost in the little streets of Vieux Nice and why not already pick a restaurant & book a table to return to later in the day?
Shoppers will want to head towards Place Masséna and from there either head down Avenue Jean Medecin or west toward the more high-end Rue de Verdun and rue de Paradis.
Personally, I'd hunt down some Socca and/or ice cream in the old town and stroll down the promenade des Anglais.
For those who take the car and plan to be away a whole day we recommend parking at one of the Park & Ride lots along the tramline. Parking is free if you buy a return tram ticket (be careful - one way tickets won't allow you to exit the parking lot)
For shorter parking times (or if you are many in one car!) we usually recommend the parking at the harbour.