Hi, If you want lively I would think Cancun was your best bet. Check out the Riu Cancun. Riu hotels are excellent. Have fun looking!

Totally agree Riu's are the best i cannot recommend the Riu cancun enough stayed there last xmas going back to mexico this xmas but trying riu yucatan in playacar

stopped at the riu yucatan last year, the other riu hotels are a walking distance away and can be visited for a choice of evening entertainment.
you can walk into playacarmen strip which has a variety of bars and restaurants.
while there we hired a minibus and visited the riu cancun(1 hour away),we were most welcomed when we showed our riu a/i wriststrap,while stopping for dinner and a few drinks.
our impression of the riu cancun was more of a party feel the american students were a touch rowdy around the pool area but all in good fun,of course you also had the benefit of going into cancun at night of which i cannot comment.the yucatan had a more laid back feel but both hotels were perfectly on the beach if you wanted a quieter area.