Marina Sabina hostel is really nice too - not partyparty (but still party
) like Juan but they have a rooftop hangout that is excellent.
Although I would skip Playa del Carmen - it's waaay overprized on everything (forget about getting an economical meal in a resto here) and isn't that much fun, to be honest.
I recommend Isla Mujeres for party and lazy beach hangout (only place in Yucatan where you can see the sun set over water: http://www.facebook.com/photo.php?pid=1334666&l=2b7e3&id=536472131
There are two hostels here, PocNa (the legend) and a new hostel that just opened up: Hi-Na-Ha V.I.P. Hostel and Villas . which is pure luxury for hostellers! They offer free pickup at the ferry.
Don't miss Isla Mujeres!
If you wanna go down South I suggest Tulum, and just go through Playa del Carmen.
Have a great time!