Downtown (El Centro) is the actual backbone of Cancún. What it lacks in history, it makes up for being a true melting pot--an exciting mix of the old traditional Mexico and an ultra-modern urban sprawl.
Avenida Tulum, the main street, passing through the heart of the city, is easy to reach by public transport. The street is lined with shops, banks, internet cafes, bars, and restaurants. Be sure to stop by Parque de las Palapas, on the market square, where locals come to chat and watch the entertainment provided by various local groups. The lively atmosphere and the sound of Mexican music is infectious, and any worries will quickly be postponed to "mañana". Browse around the artisan stalls and try some of the local sweet delicacies or visit the "Hippie Market" around the corner. In the evenings, the place to be is Avenida Yaxchillan with its lively restaurants and bars, popular with locals and tourists alike.