Cycling southeast or southwest out of Paris there's some nice routes, but straight south it's nothing attractive for the first 10-15km or so out of Paris proper. The river route south along the Seine is pretty ordinary, and takes you through places like Villeneuves-Saint-Georges, which are interesting if you want to see a bit of the poorer side of suburban Paris, but anything but scenic. The cycle route that goes from Vanves/Malakof down to Massy (where it swings east), goes though nice-ish park bits nearly all the way, but it's very broken up and slow, full of walkers, prams and pushchairs, so will take you ages. Most scenic way out would be southwest, you can cycle straight up the Champs Elysees to the Bois de Boulogne, through there and through Parc de St Cloud to Versailles, and down through the big national park area, Haute Vallee, but that's a bit of a detour from straight south. If you want south, I'd say just pick a not too busy road and follow it straight south for 15-20km until you hit forest or farmland.
And agree with Skinny, do a bit of planning, check out roads on street view to give you an idea.
Cycling southeast or southwest out of Paris there's some nice routes, but straight south it's nothing attractive for the first 10-15km or so out of Paris proper. The river route south along the Seine is pretty ordinary, and takes you through places like Villeneuves-Saint-Georges, which are interesting if you want to see a bit of the poorer side of suburban Paris, but anything but scenic. The cycle route that goes from Vanves/Malakof down to Massy (where it swings east), goes though nice-ish park bits nearly all the way, but it's very broken up and slow, full of walkers, prams and pushchairs, so will take you ages. Most scenic way out would be southwest, you can cycle straight up the Champs Elysees to the Bois de Boulogne, through there and through Parc de St Cloud to Versailles, and down through the big national park area, Haute Vallee, but that's a bit of a detour from straight south. If you want south, I'd say just pick a not too busy road and follow it straight south for 15-20km until you hit forest or farmland.
And agree with Skinny, do a bit of planning, check out roads on street view to give you an idea.