Le Ciel Est à Tout le Monde 10 rue Gay-Lussac, 5e. RER Luxembourg. Mon-Sat 10am-7pm; closed Mon in Aug. 
The best kite shop in Europe. It also sells frisbees, boomerangs, etc, plus books and traditional toys. 

Nomades 37 bd Bourdon, 4e. Mº Bastille. Mon-Fri 11am-7pm, Sat & Sun 10am-7pm. 
The place to buy and rent rollerblades and equipment, with its own bar out back where you can find out about the scene. 

Au Vieux Campeur 48 rue des Écoles, 5e. Mº Maubert-Mutualité. Mon-Fri 10.30am-7.30pm, Wed till 9pm, Sat 9.30am-8pm. 
Maps, guides, climbing, hiking, camping, ski gear, plus a kids' climbing wall.