Rippling along the border between Switzerland and France is this 1780-sq-km regional natural park, a peaceful, under-explored slice of the Jura Mountains. Cyclists and walkers arrive in summer to ply trails in and around Mijoux village; Télésiège Val Mijoux (1320m) is a good starting point. In winter, Station des Rousses ski area has downhill pistes to suit beginners and 200km of dreamy, tree-fringed cross-country routes.
Public transport links are extremely thin, so it's best to have your own wheels.
There are infrequent local buses within the park year-round (see www.jurago.fr), while free navette (shuttle bus) services link some villages during ski season (roughly mid-December to early April).