I recently spent three nights in Cannes on a work trip, and so most of what I saw was the inside of meeting rooms and sales stands. However, we did have enough down-time to amuse ourselves once or twice and so that, combined with a previous short visit, has meant I think I've just about got enough to cobble together a 24 hour guide...
Depending on how you are arriving into Cannes, the best time is mid-morning when the sun is up but hasn't fully risen yet - the streets are quiet aside from joggers and dog-walkers and the roads are quiet, making it a much more peaceful place than the rest of the day. Take some time to wander along the beach-front, admiring both the waves and the huge amounts of small dogs.
12pm Lunch: Le Vesuvio*
Le Vesuvio is located on the Croisette, near the Martinez and opposite the Miramar Plage. It is an Italian-focused restaurant with a huge menu ranging from pizza and pasta to seafood and steaks, all at very reasonable (for Cannes) prices. I ate there once during my recent trip, and had veal escalope in a mushroom cream sauce, with frites and creamed spinach. Quite heavy for a lunch (!) but perfect for dinner after being on your feet all day. However, a lot of the salads and lighter options looked equally tasty so don't let this put you off a lunch visit!
*Booking Recommended for dinner reservations, not as necessary for lunch
2pm Post-lunch: Walk off those calories
You absolutely cannot go to Cannes and fail to see the ridiculous excess of it all. From ridiculous menu prices to dog-carrying designer handbags, money.is.everywhere. But, that's part of the charm of Cannes - it's a very unique place and if this level of excess makes you feel a bit unwell then you probably wouldn't be there! The Croisette is lined by designer shops, so take your time wandering up and down playing the 'guess how much that costs' game. The answers will range from the sublime to the ridiculous.
4pm Afternoon / early evening: The Carlton
The Carlton is probably the most well-known hotel in Cannes, and any visit should include a little stop-off here. Eating in the majority of restaurants in Cannes is pretty bank-busting, so my recommendation would be to splash a little less cash, and just go here for a round of drinks. If the sun is out (it probably will be) you should take advantage of the outside seating area and sip your cocktails while taking in the amazing sea-view. If you time this right as well then you can have your drink whilst watching the beautiful sunset - so much of Canens is a bit gaudy you really have to make the most of the beautiful sights when you can!
7pm Dinner: Le Petit Paris*
Le Petit Paris is located just off the Croisette on the Rue des Belges, but is definitely worth the walk. The restaurant serves class French fair but is still imaginative and interesting - I had the "unmissable" cheeseburger, which was topped with foie gras, and as boring as that sounds it was absolutely delicious and I would struggle to not have it again if I went back. More expensive than Le Vesuvio but still quite reasonable (again, for Cannes!) I would wholeheartedly recommend venturing away from the main street and hunting this out out. When we visited the staff were happy to let us while away our time until late, and we never felt harassed to leave - keep the wine flowing and spend your night in a part of Cannes that actually feels quite French.
9am Breakfast: Hunt out a Hotel
Most of the hotel restaurants in Cannes are glam, decadent and expensive. The food is all very lovely, but for the price my recommendation would be to skip eating lunch or dinner here, and instead stick with the breakfast and spend less of a fortune. If you're not staying in one of the top-quality hotels (The Carlton, The Majestic, The Grand, The Martinez etc...) then I would really recommend taking a little walk to one of the big players and experiencing one of their breakfasts. In addition to the food you will get to experience the sheer grandness of a lot of these hotels, and maybe even spot a little celeb or two.
I flew to Cannes on BA for the outward journey, and Easyjet for the inward.