What you do with a day in Toulon will very much depend on when you go. Winters are moderate and dry, but, this being the south of France, summers can be extremely hot, so you might be inclined to chill out at one of the nearby beaches. If so, you'll need to hire a car as the best beaches, Les Sablettes in la Seyne sur Mer, L'Argentière, and La Capte, near Hyères (a good beach for children) are all outside town. It would be a pity to set out for the beach without seeing what Toulon has to offer, so why not do both? Spend the morning sightseeing, spend the afternoon at the beach and then sample some nightlife in the evening.
See it all in a morning
Perhaps the easiest way to see everything quickly is to hop aboard the tourist train. There are two separate tours: one takes you to the beach at Mourillon and the fortifications at Royal Tour; the other journey will take you around the naval base and to the most interesting sites in the town. On Cours Lafayette, opposite the marina, stop off for Toulon Market, one of the most famous in all Provence and worth seeing for its selection of locally produced fruit, vegetables, flowers, and textiles. It's open every day except Monday.
Steeped in naval history
Toulon used to be an important military port, largely due to the protection afforded to the port by the Saint-Mandrier peninsula, so a boat excursion of the calm waters is probably the best way to explore the bay. There are several organised tours, on which you can learn all about the history of the port's fortifications and the various military vessels harboured there, as well as pay a visit to the mussel farm at Lazaret Bay.
If you're in the mood to find out more, pay a visit to the National Maritime Museum. Set in the grand surroundings of the 
Toulon arsenal, it was constructed in 1738 and recently renovated to improve visitors' understanding of Toulon's long naval history.
Getting arty
A strong naval tradition is not the only thing Toulon is known for. In University Avenue, you'll come across the Museum of Art, itself a work of art built in the Italian Renaissance style. Inside, there are many collections 
from the same era, mostly paintings depicting the Provence countryside. Modern artists, such as Arman, Christo, and Klein have not been overlooked either. For something less traditional, the Asian Art Museum at Mourillon displays works from the Chinese dynasties, as well as from Southeast Asia, India, Tibet and Mongolia.
A perfect end to a perfect day
To finish off a great day, why not combine dinner with some live entertainment at the Café Théatre de la Porte d'Italie. Although the venue often plays host to comedies, the management takes the food seriously. Alternatively, the open-air setting of the amphitheatre at Châteauvallon makes the theatrical performances here in the warm evenings all the more memorable.
For a meal out, there are numerous restaurants in the harbour. Pizze Du Porte has earned a reputation for fantastic pizza, while Grande Café de la Rade offers more traditional French fare with a choice of eating inside or al fresco.
To enjoy Toulon at its most atmospheric, it's a good idea to visit during the jazz festival that's held in the town each year, but no matter how short your visit, you'll always find plenty to do in Toulon.