There is nothing like the Cinque Terre on the Riviera, but there are many smaller towns that are charming and offer a good mix of hiking, beaches etc. You just have to determine what mix of activities you want. Once you get west of St. Tropez, things get smaller, very charming, and uncrowded. They also get cheaper and quieter... perhaps too much for young folks. For hiking, you can stay in a sea side town and use the excellent system of littoral paths that most communities along the cote D'Azur have to offer. In many cases, these are converted rail lines. In others, they are walks along the season (e.g. Sentier du Littoral in Boulouris/ St. Raphael). There is more hard-core hiking in the canyons of the back-country (e.g. Gorges Du Verdon... check it out), but you have to get there and it takes planning and time with public transportation.
As you get closer to Spain, you also get a lot of point-to-point hiking as you get part of the St. Jean De Compostelle network of trails and gites.
Again, it all depends what you want. St.Tropez (there is TER train + bus service there from Nice via St. Raphael, as well as ferry service) is very charming, but it is also the playground of the rich and famous and lodging is likely to be very expensive compared to other locales. But it is charming. There is a littoral path there too.
Smaller towns, such as Rayol-Canadel-sur -Mer for example, are much smaller and in many ways more charming. Rayol, in particular has gorgeous beaches and cystal clear water, and also has miles of hiking paths, both flat and strenous ascents. It just doesn't have a town like St. Tropez, Ste. Maxime, or Le Lavandou (the nearest larger town to Rayol).
In view of all this charm, I cannot recommend bigger cities like Nice or even Cannes, as they are crowded and noisy. And Marseille is no-go... it remains a dangerous and gritty city. Toulon remains very French and is more charming than the others, but it too is gritty. Lots to choose from.
