i like the Riu resorts
cabos really nice, cozumel and la playa del carmen are also both a good time. the weather in cabo will be hot and dry (it only rains like 3 days a year there), and the other 2 will be hot and a little more humid.
anything for 8 days you will pay around 1400+ this year for a nice all inclusive. the prices jumped up again this year. 3 years ago the same vacation would have only cost 1100 
anywho, cabo is my first choice, it was the cleanest of all the mexican citys/towns ive been to, and the diving is pretty good. not a lot of coral, but a ton of fish and sea lions and shit.
if you go early enough (mating seasion) you can go whale watching too.