With all its hills, cobbled lanes and questionable road-saftey Lisbon city-centre isn’t the best place for getting around by bike but there are free bike rental booths around Cascais to explore the coastline. One by the train station, one along the coast, near Casa da Guia, and one in front of the Citadel. You need a passport or ID card.
From Cascais there’s a flat cycle path to Guincho Beach. Also, Cycling Through The Centuries (tel: +351 21 242 4604; www.cycling-centuries.com) offer mountain biking treks throughout the region and bike rental.