And once down in Cannes, where else is worth visiting down there (Geekman you prob have a raft of info for me here!

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Cheers!
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For the routes, I personally take the boring option: stick Waze on the phone and go on the toll roads. It just gets too long and difficult otherwise, unless you're willing to make the journey last several days.
As for when you're down there, it depends how much driving you want to do.
Cannes and Antibes are lovely, but will be absolutely rammed in the summer. My personal favourite places along the Riviera are as follows:
Menton: I might be biased as this is my local town, but it really is lovely. The beaches are big enough that you can always find a nice spot, the town is beautiful, inexpensive, and there's loads of great restaurants on the seafront.
Monaco: Obviously, extremely busy in the summer, but I think it's totally worth it. It's like no other place I've ever been to - I currently spend most of my days working there and every time I go walk/drive around I'm still amazed by it. If you go there, here's a bit of a tip: on the way in you'll see signs for various carparks, which will most likely be full in the summer. Ignore them all and follow the signs to the casino. Once you're near, you'll see green signs for casino carpark. There's almost always spaces and it's very cheap - I sometimes park there for a few hours when I'm working and it's usually only about €6.
Nice: Great place to spend an evening. Amazing bars, restaurants and buildings - sure, it gets busy, but it's a big enough place that it never gets too much. There's parking for most of the way along the promenade d'anglais, keep driving and you're guaranteed to find a place. There's also some underground carparks, but they're expensive.
Ile de Porquerolles: This is a bit of a trek for you - you get the boat from Hyeres. Personally, I'd say it's totally worth it: the beaches are the best I've ever been to, and the whole island is an amazing experience. It's only accessible as a foot passenger on the boat, so there's no cars on the island which makes it quite unique. It gets busy in the summer, but if you're willing to walk (or hire a bike at an extortionate rate) you can access some virtually deserted beaches.
Sospel and Breil-sur-Roya: These are up in the mountains, next to the italian border - if you drive there, you'll probably go past my flat! They're beautiful villages - great architecture and stunning mountain scenery. The roads around them are also brilliant - wide, empty, and normally deserted.
Some might disagree with me, but I wouldn't bother visiting St Tropez. It's nice enough, but it's a lot smaller than people think, extremely expensive (for eating out I'd say more expensive than Monaco) and there's not actually a huge amount to do there.
Hope that helped. If you want to know anything else, let me know!