Do lots of things. You can do lots of things coming solo to Cancun, there are nightclubs, hostels where you can find lots of people from anywhere. During the day go to playa norte in isla Mujeres, go to Puerto Morelos for a relaxing day. Most important reserve your transportation prior to arrival at the airport, since there is only taxis that would charge rates starting at $50 USD or the shuttle bus that would take for about 10 USD. For the same price you can reserve something online. For example with www.cancunairporttransportations.com
Hey, I have a new one to do solo! Go Diving, I just went diving with Diving Center Cancun, I did 4 dives in two days, I went to the MUSA Underwater Museum, Manchones Reef, Punta Negra Reef and a current reef the last one, I had a great time so I want to invite you to do the same! Visit them at www.divingcentercancun.com