Belize?
I spent almost 2 weeks in Playa Del Carmen (been home 2 days now)
it's got the Ferry to Cozumel, so you can get your dives in there or even off Playa (plenty of snorkeling as well (starfish pool on the SW corner of the island)
if you like to shop, Playa has 87 metric shit tones of shops.
as for things to do... we went to the Ruins in Coba, Tulum, Chichen-Itza, San Grevisio (Cozumel) (there are more in Cozumel and on the mainland)
Cenotes you can swim in (and dive) are all over the place
there are theme parks north / south of Playa, jungle adventures (ATV, Zip Lines and Cenotes)
Dolphin encounters
etc..
There's a good time, if you stay away from the shopping on 5th ave in Playa (the barkers can be real tiresome)
IM me if you have any questions, especially if you consider any of the "tours" you can take while there.