The Mexico City government rents commuter bikes to visitors on a daily and weekly basis. You'll need a Visa or MasterCard for the deposit and a passport or driver's license for ID (you don't need to give it over). The bicycle-share program works with smart cards and allows you to ride for up to 45 minutes between docking stations.
To avoid paying a fine for exceeding 45 minutes, simply exchange the bike at a different station. Ecobici is a great option for exploring the Centro, Roma, Condesa, and Zona Rosa and Reforma neighborhoods, which have the greatest concentration of stations. You can also register in front of the plaza Reforma 222 at a booth with the same hours on Paseo de la Reforma 222.